Sql queries: Difference between revisions
From wikinotes
Line 34: | Line 34: | ||
The example below finds <code>users.id</code> values<br> | The example below finds <code>users.id</code> values<br> | ||
that are not present in the many:many <code>user_departments</code> table,<br> | that are not present in the many:many <code>user_departments</code> table,<br> | ||
by joining a user table to itself. | by joining a user table query to itself. | ||
<syntaxhighlight lang="MySQL"> | <syntaxhighlight lang="MySQL"> | ||
Line 45: | Line 45: | ||
INNER JOIN users ON users.id = user_departments.user_id | INNER JOIN users ON users.id = user_departments.user_id | ||
) AS users_with_departments | ) AS users_with_departments | ||
ON users_with_departments.user_id = users.id | |||
WHERE users_with_departments.user_id IS NULL; | WHERE users_with_departments.user_id IS NULL; |
Revision as of 19:00, 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 finds
users.id
values
that are not present in the many:manyuser_departments
table,
by joining a user table query 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 ON users_with_departments.user_id = users.id 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