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MySQL | WHERE

schedule Mar 5, 2023
Last updated
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MySQL
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The WHERE clause allows us to add conditions to our query. Only records that meet our condition will be retrieved.

MySQL supports the following conditions:

Condition

= (equal)

<> (not equal)

<

>

<=

>=

BETWEEN

LIKE

IN

NOTE

The formal SQL standard defines = and <> as the operators for equal and not equal. Therefore it is not recommended to use == to represent equality, and != to represent inequality.

Syntax

SELECT column_name(s)
FROM table_name
WHERE condition;

Examples

Consider the following table about students' extracurricular activities:

student_id

club

date_entered

1

Football

2016-02-13

2

Boxing

2016-05-25

3

Apple

2018-08-17

4

Fishing

2017-01-01

5

NULL

NULL

The above sample table can be created using the code here.

Basic usage

To retrieve students who were part of a club before 2017-01-01:

SELECT *
FROM extracurricular
WHERE date_entered < '2017-01-01';
+------------+----------+--------------+
| student_id | club | date_entered |
+------------+----------+--------------+
| 1 | Football | 2016-02-13 |
| 2 | Boxing | 2016-05-25 |
+------------+----------+--------------+

Only students with id 1 and 2 were part of a club before 1 January 2017.

To retrieve students who are part of the Fishing club:

SELECT *
FROM extracurricular
WHERE club = 'Fishing';
+------------+---------+--------------+
| student_id | club | date_entered |
+------------+---------+--------------+
| 4 | Fishing | 2017-01-01 |
+------------+---------+--------------+

Filter missing values

We can use the IS NOT NULL operator to filter out NULL records:

SELECT *
FROM extracurricular
WHERE club IS NOT NULL;
+------------+----------+--------------+
| student_id | club | date_entered |
+------------+----------+--------------+
| 1 | Football | 2016-02-13 |
| 2 | Boxing | 2016-05-25 |
| 3 | Chess | 2018-08-17 |
| 4 | Fishing | 2017-01-01 |
+------------+----------+--------------+

We can see the record for student_id=5 has been filtered out.

robocat
Published by Arthur Yanagisawa
Edited by 0 others
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