Sql queries: Difference between revisions
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You can nest SQL queries and treat them as tables.<br> | You can nest SQL queries and treat them as tables.<br> | ||
The example below finds <code>users.id</code> values that are not present in the 1:many <code>user_departments</code> table. | The example below finds <code>users.id</code> values that are not present in the 1:many <code>user_departments</code> table<br> | ||
by joining the user table to itself. | |||
<syntaxhighlight lang="MySQL"> | <syntaxhighlight lang="MySQL"> | ||
SELECT users.* | SELECT users.* |
Revision as of 18:57, 19 September 2021
Example
SELECT * FROM users WHERE username = "dvader" AND age > 40;
Query Components
SELECT which columns you'd like to see, and how you'd like them to be presented (order, max results, etc).
SELECT * # select all columns SELECT name, age FROM users; # select name, and age only SELECT name AS userame FROM users; # rename column in results table SELECT DISTINCT name FROM users; # select 1x row for each unique name in users SELECT * FROM users LIMIT 10; # only select first 10 results SELECT * FROM users ORDER BY name ASC # sort results in ascending order by name
- Use sql joins to associate the queried table's data to other tables.
- Use sql comparison operators in your
WHERE
statement to determine what you want to select.
Techniques
Nested Queries
You can nest SQL queries and treat them as tables.
The example below findsusers.id
values that are not present in the 1:manyuser_departments
table
by joining the user table to itself.SELECT users.* FROM users LEFT JOIN ( SELECT DISTINCT users.id AS user_id FROM user_departments INNER JOIN users ON users.id = user_departments.user_id ) AS users_with_departments WHERE users_with_departments.user_id IS NULL;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