I’ve been putting together a basic SQL cheat sheet that could be used as a reference guide. Here are a series of common procedures that should be of use for anyone who uses SQL to extract data. No explanations are provided as they should largely be known to the end user.
-- COUNT OF DISTINCT VALUES
COUNT(DISTINCT year) AS years_count
COUNT(DISTINCT month) AS months_count
-- SELECT DUPLICATED ROWS
SELECT name, email, COUNT(*)
FROM users
GROUP BY name, email
HAVING COUNT(*) > 1
SELECT name, email
FROM users
WHERE email in
(
SELECT email
FROM users
GROUP BY email
HAVING COUNT(*)>1
)
SELECT firstname, lastname, list.address FROM list
INNER JOIN (SELECT address FROM list
GROUP BY address HAVING count(id) > 1) dup ON list.address = dup.address
SELECT
td.user_id,
td.brand,
td.order_dt,
COUNT(*)
FROM training_details AS td
INNER JOIN users as u on u.user_id = td.user_id
GROUP BY 1, 2, 3
HAVING COUNT(*) >= 2
ORDER BY td.order_date DESC;
-- USING CASE WHEN - ONE
SELECT rr.id,
rr.created_at,
rr.report_id,
rr.account_id,
rr.executed_by_id,
rr.data_source_id,
rr.state,
MAX(CASE WHEN rs.id IS NOT NULL OR lsr.id IS NOT NULL THEN 1 ELSE 0 END) AS scheduled_run
FROM report_runs rr
LEFT JOIN report_schedule_runs rs ON rs.report_run_id = rr.id
LEFT JOIN list_run_report_runs lrrr ON lrrr.report_run_id = rr.id
LEFT JOIN list_schedule_runs lsr ON lsr.list_run_id = lrrr.list_run_id
GROUP BY 1,2,3,4,5,6,7
-- SEE THE FIRST FEW ROWS OF A TABLE
SELECT count(1) FROM table;
SELECT * FROM table LIMIT 5;
-- USING THE LIKE OPERATOR
SELECT * FROM student WHERE name LIKE ‘d%n’;
(returns dan or den)
-- CHECKING QUERY PERFORMANCE
EXPLAIN QUERY PLAN SELECT * FROM student;
-- SUBQUERY - ONE
SELECT SUM (Sales) FROM Store_Information
WHERE Store_Name IN
(SELECT Store_Name FROM Geography
WHERE Region_Name = 'West');
-- SUBQUERY - TWO
SELECT SUM (a1.Sales) FROM Store_Information a1
WHERE a1.Store_Name IN
(SELECT Store_Name FROM Geography a2
WHERE a2.Store_Name = a1.Store_Name);
-- SUBQUERY - TWO
SELECT sub.*
FROM (
SELECT *
FROM table
WHERE day_of_week = 'Friday'
) sub
WHERE sub.resolution = 'NONE'
-- SUBQUERY - THREE
SELECT *
FROM table
WHERE Date IN (SELECT date
FROM table
ORDER BY date
LIMIT 5
)
-- SUBQUERY - FOUR
SELECT incidents.*,
sub.incidents AS incidents_that_day
FROM tutorial.sf_crime_incidents_2014_01 incidents
JOIN ( SELECT date,
COUNT(incidnt_num) AS incidents
FROM tutorial.sf_crime_incidents_2014_01
GROUP BY 1
) sub
ON incidents.date = sub.date
ORDER BY sub.incidents DESC, time
-- SELECT RECORDS FROM A TIME FRAME
SELECT * FROM users WHERE TO_DAYS(last_login) = ( TO_DAYS(NOW()) - 1 )
SELECT* FROM users DATE_SUB(NOW(),INTERVAL 90 MINUTE);
-- UNION OPERATOR
SELECT users.name
FROM users WHERE (users.name BETWEEN 'A%' AND 'M%')
UNION
SELECT banned_users.name FROM banned_users
WHERE (banned_users.name BETWEEN 'A%' AND 'M%');
-- CONCATENATE DATA INTO ONE COLUMN
SELECT CONCAT(emp.firstname, '-', emp.lastname) AS emp_full_name FROM emp;
-- INDEXING STRINGS
SELECT LEFT(date, 10) AS cleaned_date,
RIGHT(date, 17) AS cleaned_time
FROM table
SELECT SUBSTR(date, 4, 2) AS day
FROM table
-- OTHER
Select database: use [database];
Show all tables: show tables;
Show table structure: describe [table];
Counting and selecting grouped records:
SELECT *, (SELECT COUNT([column]) FROM [table]) AS count
FROM [table]
GROUP BY [column];
Select records containing [value]:
SELECT * FROM [table]
WHERE [column] LIKE '%[value]%';
Select records starting with [value]:
SELECT * FROM [table]
WHERE [column] LIKE '[value]%';
Select records starting with val and ending with ue:
SELECT * FROM [table]
WHERE [column] LIKE '[val_ue]';
Select a range:
SELECT * FROM [table]
WHERE [column] BETWEEN [value1] and [value2];
Select with custom order and only limit:
SELECT * FROM [table]
WHERE [column]
ORDER BY [column] ASC
LIMIT [value];
-- INNER, LEFT, RIGHT, AND OUTER JOIN
INNER JOIN: returns rows when there is a match in both tables.
LEFT JOIN: returns all rows from the left table, even if there are no matches in the right table.
RIGHT JOIN: returns all rows from the right table, even if there are no matches in the left table.
FULL JOIN: returns rows when there is a match in one of the tables.
SELF JOIN: is used to join a table to itself as if the table were two tables, temporarily renaming at least one table in the SQL statement.
CARTESIAN JOIN: returns the Cartesian product of the sets of records from the two or more joined tables.
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