Sql queries: Difference between revisions
From wikinotes
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(→SELECT) |
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Distinct only returns one result per unique value of N | Distinct only returns one result per unique value of N | ||
<syntaxhighlight lang="MySQL"> | <syntaxhighlight lang="MySQL"> | ||
SELECT DISTINCT username | SELECT DISTINCT username FROM users WHERE last_name = "skywalker"; | ||
FROM users | |||
WHERE last_name = "skywalker"; | |||
</syntaxhighlight> | </syntaxhighlight> | ||
</blockquote><!-- DISTINCT --> | </blockquote><!-- DISTINCT --> | ||
=== CONCAT === | === CONCAT === | ||
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CONCAT lets you combine columns into one | CONCAT lets you combine columns into one | ||
<syntaxhighlight lang="MySQL"> | <syntaxhighlight lang="MySQL"> | ||
SELECT CONCAT(first_name, " ", last_name) AS fullname | SELECT CONCAT(first_name, " ", last_name) AS fullname FROM users; | ||
FROM users; | |||
</syntaxhighlight> | </syntaxhighlight> | ||
</blockquote><!-- CONCAT --> | </blockquote><!-- CONCAT --> | ||
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</syntaxhighlight> | </syntaxhighlight> | ||
</blockquote><!-- SUBSTRING_INDEX --> | </blockquote><!-- SUBSTRING_INDEX --> | ||
=== LIMIT === | |||
<blockquote> | |||
LIMIT lets you choose only the first N results | |||
<syntaxhighlight lang="MySQL"> | |||
SELECT * FROM users LIMIT 10 | |||
</syntaxhighlight> | |||
</blockquote><!-- LIMIT --> | |||
=== ORDER BY === | |||
<blockquote> | |||
<syntaxhighlight lang="MySQL"> | |||
SELECT * FROM users ORDER BY first_name ASC; | |||
</syntaxhighlight> | |||
</blockquote><!-- ORDER BY --> | |||
</blockquote><!-- Select --> | </blockquote><!-- Select --> | ||
Revision as of 17:44, 19 September 2021
Example
SELECT * FROM users WHERE username = "dvader" AND age > 40;
Query Components
SELECT
Determines what columns you want to return.
SELECT * # select all columns SELECT col1, col2 # select col1, and col2 only SELECT col1 AS foo # rename column in results tableDISTINCT
Distinct only returns one result per unique value of N
SELECT DISTINCT username FROM users WHERE last_name = "skywalker";CONCAT
CONCAT lets you combine columns into one
SELECT CONCAT(first_name, " ", last_name) AS fullname FROM users;SUBSTRING_INDEX
SUBSTRING_INDEX lets you split/tokenize a string
SELECT SUBSTRING_INDEX('www.mysql.com', '.', 2) ##> www.mysqlLIMIT
LIMIT lets you choose only the first N results
SELECT * FROM users LIMIT 10ORDER BY
SELECT * FROM users ORDER BY first_name ASC;JOINS
See sql: joins
WHERE
Defines the Matches that will be selected.
Basics
SELECT * FROM users WHERE name = 'Smith' AND age < 30; # AND SELECT * FROM users WHERE name = 'Smith' OR age < 30; # OR SELECT * FROM users WHERE age BETWEEN 30 AND 40; # BETWEEN SELECT * FROM users WHERE name IN ('john', 'jane', 'iggy'); # IN SELECT * FROM users WHERE name NOT IN ('john', 'jane', 'iggy'); # NOT IN SELECT * FROM users WHERE first_name LIKE "Andr%"; # LIKE (glob match) SELECT * FROM users WHERE first_name REGEXP "^A.*" # REGEXP
Techniques
Nested Queries
SELECT user_Id FROM ( SELECT DISTINCT d.user_Id , CONCAT(u.firstName, " ", u.lastName) user_fullName FROM userDepartmentTable d INNER JOIN userTable AS u ON d.user_Id = u.user_Id WHERE d.department_Id = 2 ) u WHERE u.user_fullName REGEXP 'andr%';Dynamically Defined Tables
Some databases do not implement the SQL IN operator.
Instead, you can use VALUES.SELECT * FROM ( VALUES (1), (2) ) foo(id);Most useful within a join
SELECT * FROM ( VALUES (1), (2) ) foo(id) INNER JOIN ( (VALUES (1)) bar(id) ) ON foo.id = bar.id
ORDER BY
SELECT lastName, firstName FROM customers ORDER BY lastName; # Returned results order determined by lastName
SELECT lastName, firstName FROM customers ORDER BY lastName DESC; # Returned results order determined by lastName, order inverted
SELECT orderNumber, status # Custom Sort order based on contained data
FROM orders
ORDER BY FIELD(status, 'In Process',
'On Hold',
'Cancelled',
'Resolved',
'Disputed',
'Shipped');