How does the MYSQL control flow function CASE work?

MySQL CASE statement is a flow control feature that allows us to build conditions in a query, such as a SELECT or WHERE clause. We have two CASE statement syntax

Syntax-1

CASE val
WHEN compare_val1 THEN result1
WHEN compare_val2 THEN result2
.
.
.
Else result
END

In the first syntax, if val is equal to compare_val1, the CASE statement returns result1. If val equals compare_val2, the CASE statement returns result2, and so on.

If val does not match any compare_val, the CASE statement returns the result specified in the ELSE clause.

Example

mysql> Select CASE 100
    -> WHEN 100 THEN 'It is matched'
    -> WHEN 200 THEN 'It is not matched'
    -> ELSE 'No use of this Number'
    -> END as 'Matching the Number';
+---------------------+
| Matching the Number |
+---------------------+
| It is matched       |
+---------------------+
1 row in set (0.06 sec)

Syntax 2

CASE
WHEN condition_1 THEN result1
WHEN condition_2 THEN result2
.
.
.
Else result
END

In the second syntax, the CASE statement returns result 1, result 2, etc.. If the condition is true. If all conditions are false, the CASE statement returns the result specified in the ELSE clause.

Example

mysql> Select CASE
    -> WHEN (100 = 100) THEN 'It is Matched'
    -> WHEN (200 = 100) Then 'It is Not Matched'
    -> Else 'No use of Number'
    -> END as 'Matching the Number';
+---------------------+
| Matching the Number |
+---------------------+
| It is Matched       |
+---------------------+
1 row in set (0.00 sec)

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