SQL Intro

Posted by Ryan Himmelwright on Tue, Apr 24, 2018
Tags linux, programming, dev, database, sql
Ross School of Business - University of Michigan, Ann Arbor, MI

SQL is one of those technologies that which to be everywhere, but yet… I somehow haven’t had to use it for anything in school or at work. I know, I know… it’s quite a feat. Still, the pervasiveness of SQL-like databases argues that I really should learn it. So I am.

The main purpose of this website is to document some of the tech stuff I do in my free time. This serves three purposes: 1) Organizing my thoughts into a post enhances my learning, 2) I can easily refer back to the post to refresh my memory in the future, 3) It’s a good medium to share what I’ve learned with others.

This post is a prime example, as it didn’t start out as a post, but a file of the notes I took as I learned SQL basics. As I progressed, I realized all of the accumulating information should be cleaned up and posted. So here we are. I’ve redone the examples, and turned my shorthand blurbs into sentences, but if this post still seems a bit different than previous ones (no images, more code snippets than words)… that’s because it’s my learning notes with makeup heavily applied. Enjoy.

Install Setup

I first installed mysql, by going to the mySQL download page and getting the appropriate the version for my VM (centos 7). This may differ based on Distro/DB.

I’m not going to cover any of the installation steps, as this post is more about the SQL language, not setting up the DB. Besides, I did this in mysql, but in the future I’ll likely look at using MariaDB anyway…

I do however want to note this issue I encountered, so I don’t stumble over it in the future. At first, I couldn’t get the /usr/bin/mysql_secure_installation command to run due to a permission denied error…

Eventually, I learned that the first time mysql runs, it creates a temp password in the log, located at /var/log/mysqld.log. Using that password, I was able to login.

DBs and Commands:

Conventionally, SQL commands are typed in all CAPS, and statements end with a terminating ;. The rest of this post contains some SQL functions, with examples of how they are used.

Show databases

To display all of the available databases, use the SHOW DATABASES; command. Note, some of the DBs displayed are ones generated for the database system. For example, in the output below, I only created dbCustomerInfo and dbTest:

mysql> SHOW DATABASES;
+--------------------+
| Database           |
+--------------------+
| information_schema |
| dbCustomerInfo     |
| dbTest             |
| mysql              |
| performance_schema |
| sys                |
+--------------------+
6 rows in set (0.00 sec)

Create database

To create a new database, use the CREATE DATABASE command. Simple.

mysql> SHOW DATABASES;
+--------------------+
| Database           |
+--------------------+
| information_schema |
| dbCustomerInfo     |
| dbTest             |
| mysql              |
| performance_schema |
| sys                |
+--------------------+
6 rows in set (0.00 sec)

mysql> CREATE DATABASE shotLivedDB;
Query OK, 1 row affected (0.00 sec)

mysql> SHOW DATABASES;
+--------------------+
| Database           |
+--------------------+
| information_schema |
| dbCustomerInfo     |
| dbTest             |
| mysql              |
| performance_schema |
| shotLivedDB        |
| sys                |
+--------------------+
7 rows in set (0.00 sec)

Drop a database (delete):

As easy as it was to create that database, deleting, or dropping it is just as effortless with the `DROP DATABASE command, so be careful!

mysql> DROP DATABASE shotLivedDB;
Query OK, 0 rows affected (0.00 sec)

mysql> SHOW DATABASES;
+--------------------+
| Database           |
+--------------------+
| information_schema |
| dbCustomerInfo     |
| dbTest             |
| mysql              |
| performance_schema |
| sys                |
+--------------------+
6 rows in set (0.00 sec)

Select DB

To select which db to use inside the mysql shell use… USE. For this tutorial, I created a database named dbTest and selected it with USE dbTest.

Tables

Note: make sure to first select a DB to work with: USE dbTest;

Show Tables

To show all of the tables in a database, use SHOW TABLES;. For example:

mysql> SHOW TABLES;
+------------------+
| Tables_in_dbTest |
+------------------+
| tblUsers         |
+------------------+
1 row in set (0.00 sec)

Create Table

To create a new table, use CREATE TABLE. The CREATE TABLE function takes 1) the name of the new table, and 2) a list of the table fields (with their data types). For example, to create a user information table that contains a user’s first name, last name, age, and state, as well as an identification number, the following SQL command can be used:

mysql> CREATE TABLE tblUsers (id int PRIMARY KEY AUTO_INCREMENT, firstname varchar(50),lastname varchar(50), age INT,state varchar(2));
Query OK, 0 rows affected (0.01 sec)

The first name, last name, and state columns have a varchar data type, which are strings of various sizes (50 and 2 characters in this case). The id and age columns have an int data type. Notice that the id column has some other junk defined after the int identifier….

Constraints and Fields

In addition to specifying data type, other constraints can be imposed on columns when defining a new table. Constraints are used to limit the type of data that goes into the table, and can be implemented at the column or table level. To see the fields of a table, use the SHOW FIELDS FROM tablename command:

mysql> SHOW FIELDS FROM tblUsers;
+-----------|-------------|------|-----|---------|----------------+
| Field     | Type        | Null | Key | Default | Extra          |
+-----------|-------------|------|-----|---------|----------------+
| id        | int(11)     | NO   | PRI | NULL    | auto_increment |
| firstname | varchar(50) | YES  |     | NULL    |                |
| lastname  | varchar(50) | YES  |     | NULL    |                |
| age       | int(11)     | YES  |     | NULL    |                |
| state     | varchar(2)  | YES  |     | NULL    |                |
+-----------|-------------|------|-----|---------|----------------+
5 rows in set (0.00 sec)

The id column uses the PRIMARY KEY and AUTO_INCREMENT constraints. PRIMARY_KEY is a combination of the NOT NULL and UNIQUE constraints, meaning it ensures that all values in the column are unique and not NULL. The AUTO_INCREMENT field generates an unique number that is automatically incremented during each insert to the table.

INSERT INTO

To actually add data to the table, the INSERT INTO command is used. As an example, to add some users to the table created in the previous step:

mysql> INSERT INTO tblUsers (firstname,lastname,age,state) 
VALUES ('Joe','Fry',32,'RI');
Query OK, 1 row affected (0.01 sec)

mysql> INSERT INTO tblUsers (firstname,lastname,age,state) 
VALUES ('Emily','Flanders',22,'CA');
Query OK, 1 row affected (0.01 sec)

mysql> INSERT INTO tblUsers (firstname,lastname,age,state) 
VALUES ('Tina','Oak',42,'NC');
Query OK, 1 row affected (0.00 sec)

mysql> INSERT INTO tblUsers (firstname,lastname,age,state) 
VALUES ('Bob','Builder',51,'MO');
Query OK, 1 row affected (0.01 sec)

The tblUsers table should now contain the information of the 4 users added. To check this, use SELECT * FROM tblUsers; to select everything from the tblUsers table:

mysql> SELECT * FROM tblUsers;
+----|-----------|----------|------|-------+
| id | firstname | lastname | age  | state |
+----|-----------|----------|------|-------+
|  5 | Joe       | Fry      |   32 | RI    |
|  6 | Emily     | Flanders |   22 | CA    |
|  7 | Tina      | Oak      |   42 | NC    |
|  8 | Bob       | Builder  |   51 | MO    |
+----|-----------|----------|------|-------+
4 rows in set (0.00 sec)

INSERT INTO Another Table

The INSERT INTO command combined with SELECT FROM, can insert contents of one table into another. This technique can be quite useful, providing a simple way to create quick backups.

mysql> CREATE TABLE tblUsersBackup (id int PRIMARY KEY AUTO_INCREMENT, firstname varchar(50),lastname varchar(50), age INT,state varchar(2));
Query OK, 0 rows affected (0.02 sec)

mysql> SELECT * FROM tblUsersBackup;
Empty set (0.00 sec)

mysql> INSERT INTO tblUsersBackup SELECT * FROM tblUsers;
Query OK, 4 rows affected (0.00 sec)
Records: 4  Duplicates: 0  Warnings: 0

mysql> SELECT * FROM tblUsersBackup;
+----|-----------|----------|------|-------+
| id | firstname | lastname | age  | state |
+----|-----------|----------|------|-------+
|  5 | Joe       | Fry      |   32 | RI    |
|  6 | Emily     | Flanders |   22 | CA    |
|  7 | Tina      | Oak      |   42 | NC    |
|  9 | Bob       | Builder  |   51 | MO    |
+----|-----------|----------|------|-------+
4 rows in set (0.00 sec)

Drop (delete) Table

To delete a table, the DROP command is used. For example, to delete the backup table from above, issue DROP TABLE tblUsersBackup;:

mysql> DROP TABLE tblUsersBackup;
Query OK, 0 rows affected (0.01 sec)

mysql> SELECT * FROM tblUsersBackup;
ERROR 1146 (42S02): Table 'dbTest.tblUsersBackup' doesn't exist

NOTE: Using DROP commands will delete the entire structure (including the schema), even if there is data in it. Be Careful!

SELECT

In addition to selecting all of the data in a table using SELECT *, The SELECT command can be utilized to select all sorts of combinations of the data. Here are a few examples:

– Select specific columns from the table:

mysql> SELECT firstname,lastname FROM tblUsers;
+-----------|----------+
| firstname | lastname |
+-----------|----------+
| Joe       | Fry      |
| Emily     | Flanders |
| Tina      | Oak      |
| Bob       | Builder  |
+-----------|----------+
4 rows in set (0.00 sec)

– Grab column(s), after matching in another:

mysql> SELECT firstname,lastname FROM tblUsers WHERE state="RI";
+-----------|----------+
| firstname | lastname |
+-----------|----------+
| Joe       | Fry      |
+-----------|----------+
1 row in set (0.00 sec)

In addition to searching using = (which only grabs exact matches), other operators, such as >, <, <=, >=, <> (not equal), BETWEEN (between an inclusive range), and LIKE (search for pattern) can be used with the WHERE clause. For example:

mysql> SELECT firstname,lastname FROM tblUsers WHERE age>=25;
+-----------|----------+
| firstname | lastname |
+-----------|----------+
| Joe       | Fry      |
| Tina      | Oak      |
| Bob       | Builder  |
+-----------|----------+
3 rows in set (0.00 sec)
mysql> SELECT firstname,lastname FROM tblUsers WHERE age BETWEEN 30 AND 50;
+-----------|----------+
| firstname | lastname |
+-----------|----------+
| Joe       | Fry      |
| Tina      | Oak      |
+-----------|----------+
2 rows in set (0.00 sec)

ALTER

To change the table schema (add, drop or modify columns), the ALTER command is used along with ADD, DROP COLUMN, or MODIFY COLUMN (respectively) after the tablename. For example:

– Add a born column to the table

mysql> SELECT * FROM tblUsers;
+----|-----------|----------|------|-------+
| id | firstname | lastname | age  | state |
+----|-----------|----------|------|-------+
|  5 | Joe       | Fry      |   32 | RI    |
|  6 | Emily     | Flanders |   22 | CA    |
|  7 | Tina      | Oak      |   42 | NC    |
|  9 | Bob       | Builder  |   51 | MO    |
+----|-----------|----------|------|-------+
4 rows in set (0.00 sec)

mysql> ALTER TABLE tblUsers ADD born year;
Query OK, 0 rows affected (0.07 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> SELECT * FROM tblUsers;
+----|-----------|----------|------|-------|------+
| id | firstname | lastname | age  | state | born |
+----|-----------|----------|------|-------|------+
|  5 | Joe       | Fry      |   32 | RI    | NULL |
|  6 | Emily     | Flanders |   22 | CA    | NULL |
|  7 | Tina      | Oak      |   42 | NC    | NULL |
|  9 | Bob       | Builder  |   51 | MO    | NULL |
+----|-----------|----------|------|-------|------+
4 rows in set (0.00 sec)

– Change born column from a year data type to a date

mysql> ALTER TABLE tblUsers MODIFY COLUMN born date;
Query OK, 4 rows affected (0.03 sec)
Records: 4  Duplicates: 0  Warnings: 0

– Drop the born column from the table

mysql> SELECT * FROM tblUsers;
+----|-----------|----------|------|-------|------+
| id | firstname | lastname | age  | state | born |
+----|-----------|----------|------|-------|------+
|  5 | Joe       | Fry      |   32 | RI    | NULL |
|  6 | Emily     | Flanders |   22 | CA    | NULL |
|  7 | Tina      | Oak      |   42 | NC    | NULL |
|  9 | Bob       | Builder  |   51 | MO    | NULL |
+----|-----------|----------|------|-------|------+
4 rows in set (0.00 sec)

mysql> ALTER TABLE tblUsers DROP COLUMN born;
Query OK, 0 rows affected (0.04 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> SELECT * FROM tblUsers;
+----|-----------|----------|------|-------+
| id | firstname | lastname | age  | state |
+----|-----------|----------|------|-------+
|  5 | Joe       | Fry      |   32 | RI    |
|  6 | Emily     | Flanders |   22 | CA    |
|  7 | Tina      | Oak      |   42 | NC    |
|  9 | Bob       | Builder  |   51 | MO    |
+----|-----------|----------|------|-------+
4 rows in set (0.00 sec)

NOTE: Depending on the type of SQL database being used, the MODIFY command may be known as ALTER.

Delete

All the data in a table can be removed using DELETE * FROM tblName, without deleting the table schema. However, this is almost never used because there are better commands to do that.

Specific items from the table can be removed using DELETE FROM commands:

mysql> SELECT * FROM tblUsersBackup;                                                                    
+----|-----------|----------|------|-------+
| id | firstname | lastname | age  | state |
+----|-----------|----------|------|-------+
|  5 | Joe       | Fry      |   32 | RI    |
|  6 | Emily     | Flanders |   22 | CA    |
|  7 | Tina      | Oak      |   42 | NC    |
|  9 | Bob       | Builder  |   51 | MO    |
+----|-----------|----------|------|-------+
4 rows in set (0.00 sec)

mysql> DELETE FROM tblUsersBackup WHERE age>35;
Query OK, 2 rows affected (0.01 sec)

mysql> SELECT * FROM tblUsersBackup;
+----|-----------|----------|------|-------+
| id | firstname | lastname | age  | state |
+----|-----------|----------|------|-------+
|  5 | Joe       | Fry      |   32 | RI    |
|  6 | Emily     | Flanders |   22 | CA    |
+----|-----------|----------|------|-------+
2 rows in set (0.00 sec)

Multiple criteria can also be used to specify what to delete:

mysql> SELECT * FROM tblUsersBackup;                                                                    
+----|-----------|----------|------|-------+
| id | firstname | lastname | age  | state |
+----|-----------|----------|------|-------+
|  5 | Joe       | Fry      |   32 | RI    |
|  6 | Emily     | Flanders |   22 | CA    |
|  7 | Tina      | Oak      |   42 | NC    |
|  9 | Bob       | Builder  |   51 | MO    |
+----|-----------|----------|------|-------+

mysql> DELETE FROM tblUsersBackup WHERE age>50 OR state="RI";
Query OK, 2 rows affected (0.00 sec)

mysql> SELECT * FROM tblUsersBackup;
+----|-----------|----------|------|-------+
| id | firstname | lastname | age  | state |
+----|-----------|----------|------|-------+
|  6 | Emily     | Flanders |   22 | CA    |
|  7 | Tina      | Oak      |   42 | NC    |
+----|-----------|----------|------|-------+
2 rows in set (0.00 sec)

Indexes

An index speeds up the data retrieval time of a table. However, this comes at a speed cost for updating the table. So, it is usually a good idea to only index columns or tables that are frequently searched on.

To add an index, use CREATE INDEX. (Again, may differ depending on database technology being used)

mysql> SHOW FIELDS FROM tblUsers;
+-----------|-------------|------|-----|---------|----------------+
| Field     | Type        | Null | Key | Default | Extra          |
+-----------|-------------|------|-----|---------|----------------+
| id        | int(11)     | NO   | PRI | NULL    | auto_increment |
| firstname | varchar(50) | YES  |     | NULL    |                |
| lastname  | varchar(50) | YES  |     | NULL    |                |
| age       | int(11)     | YES  |     | NULL    |                |
| state     | varchar(2)  | YES  |     | NULL    |                |
+-----------|-------------|------|-----|---------|----------------+
5 rows in set (0.00 sec)

mysql> CREATE INDEX indexTblUsers ON tblUsers (id);
Query OK, 0 rows affected (0.01 sec)
Records: 0  Duplicates: 0  Warnings: 0

Drop Table

As we already know, you can drop a whole table with DROP. However, like many of the other commands, DROP can be used with other items, like an index:

mysql> ALTER TABLE tblUsers DROP INDEX indexTblUsers;
Query OK, 0 rows affected (0.01 sec)
Records: 0  Duplicates: 0  Warnings: 0

Truncate

The truncate command will delete the data, but leave table schema the same… again, still be careful because it will delete all the data in the table.

mysql> SELECT * FROM tblUsersBackup;
+----|-----------|----------|------|-------+
| id | firstname | lastname | age  | state |
+----|-----------|----------|------|-------+
|  6 | Emily     | Flanders |   22 | CA    |
|  7 | Tina      | Oak      |   42 | NC    |
+----|-----------|----------|------|-------+
2 rows in set (0.00 sec)

mysql> TRUNCATE TABLE tblUsersBackup;
Query OK, 0 rows affected (0.03 sec)

mysql> SELECT * FROM tblUsersBackup;
Empty set (0.00 sec)

Auto Increment

An Integer value, that once assigned, auto increments by 1 everytime the table is updated.

You can auto increment a key, which is very useful for IDs in a table (as we saw earlier).all the data

mysql> SHOW FIELDS FROM tblPeople;
+-----------|-------------|------|-----|---------|----------------+
| Field     | Type        | Null | Key | Default | Extra          |
+-----------|-------------|------|-----|---------|----------------+
| id        | int(11)     | NO   | PRI | NULL    | auto_increment |
| firstname | varchar(50) | YES  |     | NULL    |                |
| lastname  | varchar(50) | YES  |     | NULL    |                |
| age       | int(11)     | YES  |     | NULL    |                |
| state     | varchar(2)  | YES  |     | NULL    |                |
+-----------|-------------|------|-----|---------|----------------+
5 rows in set (0.00 sec)

mysql> INSERT INTO tblPeople (firstname, lastname, age, state) 
VALUES ('Josh', 'Rivers', 19, "PA");
Query OK, 1 row affected (0.00 sec)

mysql> INSERT INTO tblPeople (firstname, lastname, age, state) 
VALUES ('Kim', 'Medows', 32, "CO");
Query OK, 1 row affected (0.00 sec)

mysql> SELECT * FROM  tblPeople;
+----|-----------|----------|------|-------+
| id | firstname | lastname | age  | state |
+----|-----------|----------|------|-------+
|  1 | Josh      | Rivers   |   19 | PA    |
|  2 | Kim       | Medows   |   32 | CO    |
+----|-----------|----------|------|-------+
2 rows in set (0.00 sec)

Note: The auto increment value can be manually set.

mysql> INSERT INTO tblPeople (firstname, lastname, age, state) 
VALUES ('Dan', 'Valley', 40, "NH");
Query OK, 1 row affected (0.01 sec)

mysql> SELECT * FROM  tblPeople;
+------|-----------|----------|------|-------+
| id   | firstname | lastname | age  | state |
+------|-----------|----------|------|-------+
|    1 | Josh      | Rivers   |   19 | PA    |
|    2 | Kim       | Medows   |   32 | CO    |
| 1000 | Dan       | Valley   |   40 | NH    |
+------|-----------|----------|------|-------+
3 rows in set (0.00 sec)

When setting the auto increment value manually, just be careful that it isn’t set to one that will eventually overwrite another value in the table that must be unique, or it will error.

SQL Functions

SQL functions, are SQL statements that don’t directly manipulate data, but can be use to extract other useful information from the database and tables. They are often used in conjunction with SELECT. There are a bunch of base SQL functions to use. Here is a sampling of a few:

COUNT

Shows the number of matched results returned. In this example, the number of rows in tblUsers, and then the number of users over the age of 35:

mysql> SELECT * FROM tblUsers;
+----|-----------|----------|------|-------+
| id | firstname | lastname | age  | state |
+----|-----------|----------|------|-------+
|  5 | Joe       | Fry      |   32 | RI    |
|  6 | Emily     | Flanders |   22 | CA    |
|  7 | Tina      | Oak      |   42 | NC    |
|  9 | Bob       | Builder  |   51 | MO    |
+----|-----------|----------|------|-------+
4 rows in set (0.00 sec)

mysql> SELECT COUNT(*) FROM tblUsers;
+----------+
| COUNT(*) |
+----------+
|        4 |
+----------+
1 row in set (0.00 sec)

mysql> SELECT COUNT(*) FROM tblUsers WHERE age>35;
+----------+
| COUNT(*) |
+----------+
|        2 |
+----------+
1 row in set (0.00 sec)

Average and Sum Functions

To get an average of the number values in a column, use the AVG function:

mysql> SELECT AVG(age) from tblUsers;
+----------+
| AVG(age) |
+----------+
|  36.7500 |
+----------+
1 row in set (0.00 sec)

To get a total of a column’s number values, use the SUM function:

mysql> SELECT SUM(age) from tblUsers;
+----------+
| SUM(age) |
+----------+
|      147 |
+----------+
1 row in set (0.00 sec)

Note, that select statements using functions can still be combined:

mysql> SELECT COUNT(*),AVG(age) from tblUsers;
+----------|----------+
| COUNT(*) | AVG(age) |
+----------|----------+
|        4 |  36.7500 |
+----------|----------+
1 row in set (0.00 sec)

The Like Operator

The LIKE operator can be used for matching, with wildcards (%). For example in the following searches, %S and S% yield different results because the first looks for last names which end in an “s”, and the second grabs last names which start with “s”. The last example returns names which have “er” anywhere in the last name.

Note, LIKE uses higher CPU usage. With larger data sets, try to use it on columns which are indexed if possible.

mysql> SELECT * FROM tblUsers;
+----|-----------|----------|------|-------+
| id | firstname | lastname | age  | state |
+----|-----------|----------|------|-------+
|  5 | Joe       | Fry      |   32 | RI    |
|  6 | Emily     | Flanders |   22 | CA    |
|  7 | Tina      | Oak      |   42 | NC    |
|  9 | Bob       | Builder  |   51 | MO    |
| 10 | Zach      | Scout    |   27 | NV    |
+----|-----------|----------|------|-------+
5 rows in set (0.00 sec)

mysql> SELECT * FROM tblUsers WHERE lastname LIKE '%S';
+----|-----------|----------|------|-------+
| id | firstname | lastname | age  | state |
+----|-----------|----------|------|-------+
|  6 | Emily     | Flanders |   22 | CA    |
+----|-----------|----------|------|-------+
1 row in set (0.00 sec)

mysql> SELECT * FROM tblUsers WHERE lastname LIKE 'S%';
+----|-----------|----------|------|-------+
| id | firstname | lastname | age  | state |
+----|-----------|----------|------|-------+
| 10 | Zach      | Scout    |   27 | NV    |
+----|-----------|----------|------|-------+
1 row in set (0.00 sec)

mysql> SELECT * FROM tblUsers WHERE lastname LIKE '%er%';
+----|-----------|----------|------|-------+
| id | firstname | lastname | age  | state |
+----|-----------|----------|------|-------+
|  6 | Emily     | Flanders |   22 | CA    |
|  9 | Bob       | Builder  |   51 | MO    |
+----|-----------|----------|------|-------+
2 rows in set (0.00 sec)

Views

Views create a custom filter to display a set of the data. This can be useful when defining several use cases of an application. For example, a view can saved, and then just updated, instead of recalculated, when querying for dash boards, and/or reports.

mysql> SELECT * FROM tblUsers;
+----|-----------|----------|------|-------+
| id | firstname | lastname | age  | state |
+----|-----------|----------|------|-------+
|  5 | Joe       | Fry      |   32 | RI    |
|  6 | Emily     | Flanders |   22 | CA    |
|  7 | Tina      | Oak      |   42 | NC    |
|  9 | Bob       | Builder  |   51 | MO    |
| 10 | Zach      | Scout    |   27 | NV    |
+----|-----------|----------|------|-------+
5 rows in set (0.00 sec)

mysql> CREATE VIEW myView AS SELECT COUNT(*), AVG(age), SUM(age) FROM tblUsers;
Query OK, 0 rows affected (0.01 sec)

mysql> SELECT * FROM myView;
+----------|----------|----------+
| COUNT(*) | AVG(age) | SUM(age) |
+----------|----------|----------+
|        5 |  34.8000 |      174 |
+----------|----------|----------+
1 row in set (0.00 sec)

mysql> DELETE FROM tblUsers WHERE firstname='Bob';
Query OK, 1 row affected (0.01 sec)

mysql> SELECT * FROM myView;
+----------|----------|----------+
| COUNT(*) | AVG(age) | SUM(age) |
+----------|----------|----------+
|        4 |  30.7500 |      123 |
+----------|----------|----------+
1 row in set (0.00 sec)

Joins

Inner Join

An Inner join will return the selected rows from multiple tables, when there is at least one match in each table. For example:

mysql> SELECT * FROM tblUsers;
+----|-----------|----------|------|-------+
| id | firstname | lastname | age  | state |
+----|-----------|----------|------|-------+
|  5 | Joe       | Fry      |   32 | RI    |
|  6 | Emily     | Flanders |   22 | CA    |
|  7 | Tina      | Oak      |   42 | NC    |
| 10 | Zach      | Scout    |   27 | NV    |
+----|-----------|----------|------|-------+
4 rows in set (0.00 sec)

mysql> SELECT * FROM tblUsersPts;
+----|-----------|----------|-------|--------+
| id | firstname | lastname | team  | points |
+----|-----------|----------|-------|--------+
|  1 | Joe       | Fry      | Red   |  20000 |
|  2 | Emily     | Flanders | Blue  |  17000 |
|  3 | Tina      | Oak      | Red   |  32800 |
|  4 | Bob       | Builder  | Green |  40100 |
+----|-----------|----------|-------|--------+
4 rows in set (0.00 sec)

mysql> SELECT tblUsersPts.firstname, tblUsersPts.lastname, tblUsersPts.points
    -> FROM tblUsersPts INNER JOIN tblUsers
    -> ON tblUsers.lastname=tblUsersPts.lastname 
    -> AND tblUsers.firstname=tblUsersPts.firstname;
+-----------|----------|--------+
| firstname | lastname | points |
+-----------|----------|--------+
| Joe       | Fry      |  20000 |
| Emily     | Flanders |  17000 |
| Tina      | Oak      |  32800 |
+-----------|----------|--------+
3 rows in set (0.00 sec)

Right Join

A right join will return everything in the right table, and any existing/matched items on the left. Any non-matching data will display as null.

mysql> SELECT * FROM tblUsers;
+----|-----------|----------|------|-------+
| id | firstname | lastname | age  | state |
+----|-----------|----------|------|-------+
|  5 | Joe       | Fry      |   32 | RI    |
|  6 | Emily     | Flanders |   22 | CA    |
|  7 | Tina      | Oak      |   42 | NC    |
| 10 | Zach      | Scout    |   27 | NV    |
+----|-----------|----------|------|-------+
4 rows in set (0.00 sec)

mysql> SELECT * FROM tblUsersPts;
+----|-----------|----------|-------|--------+
| id | firstname | lastname | team  | points |
+----|-----------|----------|-------|--------+
|  1 | Joe       | Fry      | Red   |  20000 |
|  2 | Emily     | Flanders | Blue  |  17000 |
|  3 | Tina      | Oak      | Red   |  32800 |
|  4 | Bob       | Builder  | Green |  40100 |
+----|-----------|----------|-------|--------+
4 rows in set (0.00 sec)

mysql> SELECT firstname, lastname, points FROM tblUsersPts 
-> RIGHT JOIN tblUsers ON tblUsers.lastname=tblUsersPts.lastname 
-> AND tblUsers.firstname=tblUsersPts.firstname;
+-----------|----------|--------+
| firstname | lastname | points |
+-----------|----------|--------+
| Joe       | Fry      |  20000 |
| Emily     | Flanders |  17000 |
| Tina      | Oak      |  32800 |
| NULL      | NULL     |   NULL |
+-----------|----------|--------+
4 rows in set (0.00 sec)

Left Join

Basically the same as a RIGHT join, but with the left table items all joining.

Full Join

A full join shows all records from both the right and left table, regardless of matching the relation records of either.

NOTE: Full outer joins do not work on mysql, but would on postresql. It is recommended to use Unions to emulate them if needed.

Unions

Unions are used to combine and concatenate SELECT statements from multiple tables.

This example doesn’t work well because it doesn’t make much sense (age isn’t the same as a point), but at least it displays what is happening during the UNION.

mysql> SELECT firstname, lastname, points FROM tblUsersPts 
UNION SELECT firstname, lastname, age FROM tblUsers;
+-----------|----------|--------+
| firstname | lastname | points |
+-----------|----------|--------+
| Joe       | Fry      |  20000 |
| Emily     | Flanders |  17000 |
| Tina      | Oak      |  32800 |
| Bob       | Builder  |  40100 |
| Joe       | Fry      |     32 |
| Emily     | Flanders |     22 |
| Tina      | Oak      |     42 |
| Zach      | Scout    |     27 |
+-----------|----------|--------+
8 rows in set (0.00 sec)

Sorting Records

Record rows can be sorted in the ascending or descending order of a column by using the ODER command and either ASC for “ascending” or DESC for “descending”. The results can be limited or trimmed using another statement, like LIMIT 1.

mysql> SELECT * FROM tblUsersPts;
+----|-----------|----------|-------|--------+
| id | firstname | lastname | team  | points |
+----|-----------|----------|-------|--------+
|  1 | Joe       | Fry      | Red   |  20000 |
|  2 | Emily     | Flanders | Blue  |  17000 |
|  3 | Tina      | Oak      | Red   |  32800 |
|  4 | Bob       | Builder  | Green |  40100 |
+----|-----------|----------|-------|--------+
4 rows in set (0.00 sec)

mysql> SELECT * FROM tblUsersPts ORDER BY points DESC LIMIT 3;
+----|-----------|----------|-------|--------+
| id | firstname | lastname | team  | points |
+----|-----------|----------|-------|--------+
|  4 | Bob       | Builder  | Green |  40100 |
|  3 | Tina      | Oak      | Red   |  32800 |
|  1 | Joe       | Fry      | Red   |  20000 |
+----|-----------|----------|-------|--------+
3 rows in set (0.00 sec)

mysql> SELECT * FROM tblUsersPts ORDER BY team,lastname ASC;
+----|-----------|----------|-------|--------+
| id | firstname | lastname | team  | points |
+----|-----------|----------|-------|--------+
|  2 | Emily     | Flanders | Blue  |  17000 |
|  4 | Bob       | Builder  | Green |  40100 |
|  1 | Joe       | Fry      | Red   |  20000 |
|  3 | Tina      | Oak      | Red   |  32800 |
+----|-----------|----------|-------|--------+
4 rows in set (0.00 sec)

Minimum and Maximum Values

Similar to ORDER, the minimum and maximum values in a particular column of the table can be returned using the MIN and MAX commands:

mysql> SELECT * FROM tblUsersPts ORDER BY team,lastname ASC;
+----|-----------|----------|-------|--------+
| id | firstname | lastname | team  | points |
+----|-----------|----------|-------|--------+
|  2 | Emily     | Flanders | Blue  |  17000 |
|  4 | Bob       | Builder  | Green |  40100 |
|  1 | Joe       | Fry      | Red   |  20000 |
|  3 | Tina      | Oak      | Red   |  32800 |
+----|-----------|----------|-------|--------+
4 rows in set (0.00 sec)

mysql> SELECT MIN(points),MAX(points)  FROM tblUsersPts;
+-------------|-------------+
| MIN(points) | MAX(points) |
+-------------|-------------+
|       17000 |       40100 |
+-------------|-------------+
1 row in set (0.00 sec)

To return other fields with the min/max item, a sub-query (another SQL query inside parenthesis) may have to be used:

mysql> SELECT firstname,lastname,points FROM tblUsersPts 
-> WHERE points=(SELECT MAX(points) FROM tblUsersPts);
+-----------|----------|--------+
| firstname | lastname | points |
+-----------|----------|--------+
| Bob       | Builder  |  40100 |
+-----------|----------|--------+
1 row in set (0.00 sec)

Upper and Lower Case Conversions

Strings (such as names), and be easily altered using functions like UCASE and LCASE. These two functions change the displayed text (the data is not altered) to be upper or lower case.

mysql> SELECT * FROM tblUsersPts;
+----|-----------|----------|-------|--------+
| id | firstname | lastname | team  | points |
+----|-----------|----------|-------|--------+
|  1 | Joe       | Fry      | Red   |  20000 |
|  2 | Emily     | Flanders | Blue  |  17000 |
|  3 | Tina      | Oak      | Red   |  32800 |
|  4 | Bob       | Builder  | Green |  40100 |
+----|-----------|----------|-------|--------+
4 rows in set (0.01 sec)

mysql> SELECT UCASE(lastname),LCASE(firstname) FROM tblUsersPts;
+-----------------|------------------+
| UCASE(lastname) | LCASE(firstname) |
+-----------------|------------------+
| FRY             | joe              |
| FLANDERS        | emily            |
| OAK             | tina             |
| BUILDER         | bob              |
+-----------------|------------------+
4 rows in set (0.00 sec)

mysql> SELECT * FROM tblUsersPts;
+----|-----------|----------|-------|--------+
| id | firstname | lastname | team  | points |
+----|-----------|----------|-------|--------+
|  1 | Joe       | Fry      | Red   |  20000 |
|  2 | Emily     | Flanders | Blue  |  17000 |
|  3 | Tina      | Oak      | Red   |  32800 |
|  4 | Bob       | Builder  | Green |  40100 |
+----|-----------|----------|-------|--------+
4 rows in set (0.00 sec)

Now()

The Now*() function creates a new value, using the current date and time. This can be appended when creating a view, to mark when changes happen over time.

mysql> SELECT id,firstname,lastname,team,points, Now() AS updated
    -> FROM tblUsersPts;
+----|-----------|----------|-------|--------|---------------------+
| id | firstname | lastname | team  | points | updated             |
+----|-----------|----------|-------|--------|---------------------+
|  1 | Joe       | Fry      | Red   |  20000 | 2018-04-17 10:56:53 |
|  2 | Emily     | Flanders | Blue  |  17000 | 2018-04-17 10:56:53 |
|  3 | Tina      | Oak      | Red   |  32800 | 2018-04-17 10:56:53 |
|  4 | Bob       | Builder  | Green |  40100 | 2018-04-17 10:56:53 |
+----|-----------|----------|-------|--------|---------------------+
4 rows in set (0.00 sec)

Conclusion

I think that is enough to at least get started with SQL :) (it was for me anyway). There’s not much else to say other than hopefully this post server as a good SQL quick-reference down the road :). Enjoy!

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