Sql queries: Difference between revisions
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Determines what columns you want to return. | Determines what columns you want to return. | ||
Basics | == Basics == | ||
<blockquote> | |||
<syntaxhighlight lang="MySQL"> | <syntaxhighlight lang="MySQL"> | ||
SELECT * # select all columns | SELECT * # select all columns | ||
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SELECT col1 AS foo # rename column in results table | SELECT col1 AS foo # rename column in results table | ||
</syntaxhighlight> | </syntaxhighlight> | ||
</blockquote><!-- Basics --> | |||
== DISTINCT == | |||
<blockquote> | |||
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"> | ||
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WHERE last_name = "skywalker"; | WHERE last_name = "skywalker"; | ||
</syntaxhighlight> | </syntaxhighlight> | ||
</blockquote><!-- DISTINCT --> | |||
== LIMIT == | |||
<blockquote> | |||
LIMIT lets you choose only the first N results | LIMIT lets you choose only the first N results | ||
<syntaxhighlight lang="MySQL"> | <syntaxhighlight lang="MySQL"> | ||
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LIMIT 10 | LIMIT 10 | ||
</syntaxhighlight> | </syntaxhighlight> | ||
</blockquote><!-- LIMIT --> | |||
== CONCAT == | |||
<blockquote> | |||
CONCAT lets you combine columns into one | CONCAT lets you combine columns into one | ||
<syntaxhighlight lang="MySQL"> | <syntaxhighlight lang="MySQL"> | ||
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FROM users; | FROM users; | ||
</syntaxhighlight> | </syntaxhighlight> | ||
</blockquote><!-- CONCAT --> | |||
== SUBSTRING_INDEX == | |||
<blockquote> | |||
SUBSTRING_INDEX lets you split/tokenize a string | SUBSTRING_INDEX lets you split/tokenize a string | ||
<syntaxhighlight lang="mySQL"> | <syntaxhighlight lang="mySQL"> | ||
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##> www.mysql | ##> www.mysql | ||
</syntaxhighlight> | </syntaxhighlight> | ||
</blockquote><!-- SUBSTRING_INDEX --> | |||
</blockquote><!-- Select --> | </blockquote><!-- Select --> | ||
Revision as of 17:38, 19 September 2021
Example
SELECT * FROM users WHERE username = "dvader" AND age > 40;
Select
Determines what columns you want to return.
Basics
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";LIMIT
LIMIT lets you choose only the first N results
SELECT * FROM users LIMIT 10CONCAT
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.mysql
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</syntaxhighlight>
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');