MySQL Cheat Sheet

MySQL is a popular open-source relational database management system known for its ease of use and scalability. Sometimes, you will need a little help while working on a project. That's why we created this MySQL Cheat Sheet.

Instructions for installing MySQL are available at: https://dev.mysql.com

Connecting to a MySQL server

Connect to a MySQL server with a username and a password using the mysql command-line client. MySQL will prompt for the password:

mysql -u [username] -p

To connect to a specific database on a MySQL server using a username and a password:

mysql -u [username] -p [database]

To export data using the mysqldump tool:

mysqldump -u [username] -p [database] > data_backup.sql

To exit the client:

quit

or

exit

For a full list of commands:

help

Creating and displaying databases

To create a database:

CREATE DATABASE zoo;

To list all the databases on the server:

SHOW DATABASES;

To use a specified database:

USE zoo;

To delete a specified database:

DROP DATABASE zoo;

To list all tables in the database:

SHOW TABLES;

To get information about a specified table:

DESCRIBE animal;

It outputs column names, data types, default values, and more about the table.

Creating tables

To create a table:

CREATE TABLE habitat (
  id INT,
  name VARCHAR(64)
);

Use AUTO_INCREMENT to increment the ID automatically with each new record. An AUTO_INCREMENT column must be defined as a primary or unique key:

CREATE TABLE habitat (
  id INT PRIMARY KEY AUTO_INCREMENT,
  name VARCHAR(64)
);
  

To create a table with a foreign key:

CREATE TABLE animal (
  id INT PRIMARY KEY AUTO_INCREMENT,
  name VARCHAR(64),
  species VARCHAR(64),
  age INT,
  habitat_id INT,
  FOREIGN KEY (habitat_id) REFERENCES habitat(id)
);

Modifying tables

Use the ALTER TABLE statement to modify the table structure.

To change a table name:

ALTER TABLE animal RENAME pet;

To add a column to the table:

ALTER TABLE animal
ADD COLUMN name VARCHAR(64);

To change a column name:

ALTER TABLE animal
RENAME COLUMN id TO identifier;

To change a column data type:

ALTER TABLE animal
MODIFY COLUMN name VARCHAR(128);

To delete a column:

ALTER TABLE animal
DROP COLUMN name;

To delete a table:

DROP TABLE animal;

Querying data

To select data from a table, use the SELECT command.

An example of a single-table query:

SELECT species, AVG(age) AS average_age
FROM animal
WHERE id != 3
GROUP BY species
HAVING AVG(age) > 3
ORDER BY AVG(age) DESC;

An example of a multiple-table query:

SELECT city.name, country.name
FROM city
[INNER | LEFT | RIGHT] JOIN country
  ON city.country_id = country.id;

Use +, -, *, / to do some basic math.

To get the number of seconds in a week:

SELECT 60 * 60 * 24 * 7; 
-- result: 604800

Aggregation and grouping

Aggregate functions

• AVG(expr) − average value of expr for the group.
• COUNT(expr) − count of expr values within the group.
• MAX(expr) − maximum value of expr values within the group.
• MIN(expr) − minimum value of expr values within the group.
• SUM(expr) − sum of expr values within the group.

To count the rows in the table:

SELECT COUNT(*)
FROM animal;

To count the non-NULL values in a column:

SELECT COUNT(name)
FROM animal;

To count unique values in a column:

SELECT COUNT(DISTINCT name)
FROM animal;

GROUP BY

To count the animals by species:

SELECT species, COUNT(id)
FROM animal
GROUP BY species;

To get the average, minimum, and maximum ages by habitat:

SELECT habitat_id, AVG(age), MIN(age), MAX(age)
FROM animal
GROUP BY habitat;

Inserting data

To insert data into a table, use the INSERT command:

INSERT INTO habitat VALUES (1, 'River'), (2, 'Forest');

You may specify the columns in which the data is added. The remaining columns are filled with default values or NULLs.

INSERT INTO habitat (name) VALUES ('Savanna');

Updating data

To update the data in a table, use the UPDATE command:

UPDATE animal
SET
  species = 'Duck',
  name = 'Quack'
WHERE id = 2;

Deleting data

To delete data from a table, use the DELETE command:

DELETE FROM animal
WHERE id = 1;

This deletes all rows satisfying the WHERE condition.

To delete all data from a table, use the TRUNCATE TABLE statement:

TRUNCATE TABLE animal;

Casting

From time to time, you need to change the type of a value. Use the CAST() function to do this. In MySQL, you can cast to these data types:

• CHAR
• NCHAR
• BINARY
• DATE
• DATETIME
• DECIMAL
• DOUBLE
• FLOAT
• REAL
• SIGNED
• UNSIGNED
• TIME
• YEAR
• JSON
• spatial_type

To get a number as a signed integer:

SELECT CAST(1234.567 AS signed);
-- result: 1235

To change a column type to double:

SELECT CAST(column AS double);

Text functions

Filtering the output

To fetch the city names that are not Berlin:

SELECT name
FROM city
WHERE name != 'Berlin';

Text operators

To fetch the city names that start with a 'P' or end with an 's':

SELECT name
FROM city
WHERE name LIKE 'P%' OR name LIKE '%s';

To fetch the city names that start with any letter followed by 'ublin' (like Dublin in Ireland or Lublin in Poland):

SELECT name
FROM city
WHERE name LIKE '_ublin';

COncatenation

Use the CONCAT() function to concatenate two strings:

SELECT CONCAT('Hi ', 'there!');
-- result: Hi there!

If any of the string is NULL, the result is NULL:

SELECT CONCAT(Great ', 'day', NULL);
-- result: NULL

MySQL allows specifying a separating character (separator) using the CONCAT_WS() function. The separator is placed between the concatenated values:

SELECT CONCAT_WS(' ', 1, 'Olivier', 'Norris'); 
-- result: 1 Olivier Norris

Other useful text functions

To get the count of characters in a string:

SELECT LENGTH('LearnSQL.com');
-- result: 12

To convert all letters to lowercase:

SELECT LOWER('LEARNSQL.COM');
-- result: learnsql.com

To convert all letters to uppercase:

SELECT UPPER('LearnSQL.com');
-- result: LEARNSQL.COM

To get just a part of a string:

SELECT SUBSTRING('LearnSQL.com', 9);
-- result: .com

SELECT SUBSTRING('LearnSQL.com', 1, 5);
-- result: Learn

To replace a part of a string:

SELECT REPLACE('LearnSQL.com', 'SQL', 'Python');
-- result: LearnPython.com

Numeric functions

To get the remainder of a division:

SELECT MOD(13, 2); 
-- result: 1

To round a number to its nearest integer:

SELECT ROUND(1234.56789); 
-- result: 1235

To round a number to three decimal places:

SELECT ROUND(1234.56789, 3);
-- result: 1234.568

To round a number up:

SELECT CEIL(13.1); 
-- result: 14

SELECT CEIL(-13.9); 
-- result: -13

The CEIL(x) function returns the smallest integer not less than x.

To round the number down:

SELECT FLOOR(13.8); 
-- result: 13

SELECT FLOOR(-13.2); 
-- result: -14

The FLOOR(x) function returns the greatest integer not greater than x.

To round towards 0 irrespective of the sign of a number:

SELECT TRUNCATE(13.56, 0); 
-- result: 13

SELECT TRUNCATE(-13.56, 1); 
-- result: -13.5

To get the absolute value of a number:

SELECT ABS(-12); 
-- result: 12

To get the square root of a number:

SELECT SQRT(9); 
-- result: 3

Useful NULL functions

To fetch the names of the cities whose rating values are not missing:

SELECT name
FROM city
WHERE rating IS NOT NULL;

COALESCE

To replace NULL in a query with something meaningful:

SELECT domain, COALESCE(domain, 'domain missing')
FROM contacts;

The COALESCE() function takes any number of arguments and returns the value of the first argument that is not NULL.

NULLIF

To save yourself from division by 0 errors:

SELECT last_month, this_month,
  this_month * 100.0 / NULLIF(last_month, 0) AS better_by_percent
FROM video_views;

The NULLIF(x, y) function returns NULL if x equals y, else it returns the value of x value.

Date and time

There are 5 main time-related types in MySQL: DATE, TIME, DATETIME, TIMESTAMP, YEAR.

Intervals

An interval is the duration between two points in time.

To define an interval:

INTERVAL 1 DAY

This syntax consists of the INTERVAL keyword, a value, and a time part keyword (YEAR, QUARTER, MONTH, WEEK, DAY, HOUR, MINUTE, SECOND, MICROSECOND).

You may combine different INTERVALs using the + or - operator:

INTERVAL 1 YEAR + INTERVAL 3 MONTH

You may also use the standard SQL syntax:

INTERVAL '1-3' YEAR_MONTH
-- 1 year and 3 months

INTERVAL '3-12' HOUR_MINUTE
-- 3 hours 12 minutes

What time is it?

To answer this question, use:

• CURRENT_TIME or CURTIME – to get the current time.
• CURRENT_DATE or CURDATE – to get the current date.
• NOW() or CURRENT_TIMESTAMP – to get the current timestamp with both of the above.

Creating values

To create a date, time, or datetime, write the value as a string and cast it to the proper type.

SELECT CAST('2021-12-31' AS date), 
  CAST('15:31' AS time), 
  CAST('2021-12-31 23:59:29' AS datetime);

You may skip casting in simple conditions; the database knows what you mean.

SELECT airline, flight_no, departure_time
FROM airport_schedule
WHERE departure_time < '12:00';

Extracting parts of dates

To extract a part of a date, use the functions YEAR, MONTH, WEEK, DAY, HOUR, and so on.

SELECT YEAR(CAST('2021-12-31' AS date));
-- result: 2021

SELECT MONTH(CAST('2021-12-31' AS date));
-- result: 12

SELECT DAY(CAST('2021-12-31' AS date));
-- result: 31

Date arithmetics

To add or subtract an interval from a DATE, use the ADDDATE() function:

ADDDATE('2021-10-31', INTERVAL 2 MONTH);
-- result: '2021-12-31'

ADDDATE('2014-04-05', INTERVAL -3 DAY);
-- result: '2014-04-02'

To add or subtract an interval from a TIMESTAMP or DATETIME, use the TIMESTAMPADD() function:

TIMESTAMPADD(MONTH, 2,'2014-06-10 07:55:00');
-- result: '2014-08-10 07:55:00'

TIMESTAMPADD(MONTH, -2, '2014-06-10 07:55:00');
-- result: '2014-04-10 07:55:00'

To add or subtract TIME from a DATETIME, use the ADDTIME() function:

ADDTIME('2018-02-12 10:20:24', '12:43:02');
-- result: '2018-02-12 23:03:26'

ADDTIME('2018-02-12 10:20:24', '-12:43:02');
-- result: '2018-02-11 21:37:22'

To find the difference between two dates, use the DATEDIFF() function:

DATEDIFF('2015-01-01', '2014-01-02');
-- result: 364

To find the difference between two times, use the TIMEDIFF() function:

SELECT TIMEDIFF('09:30:00', '07:55:00');
-- result: '01:35:00'

To find the difference between two datetimes (in a given unit of time), use the TIMESTAMPDIFF() function. Here's an example with the difference given in weeks:

SELECT TIMESTAMPDIFF(WEEK, '2018-02-26', '2018-03-21'); 
-- result: 3