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control-flow-functions: Update reference manual links (#16849)
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‎functions-and-operators/control-flow-functions.md

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# Control Flow Functions
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TiDB supports all of the [control flow functions](https://dev.mysql.com/doc/refman/5.7/en/flow-control-functions.html) available in MySQL 5.7.
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TiDB supports all of the [control flow functions](https://dev.mysql.com/doc/refman/8.0/en/flow-control-functions.html) available in MySQL 8.0.
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| Name | Description |
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|:--------------------------------------------------------------------------------------------------|:----------------------------------|
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| [`CASE`](https://dev.mysql.com/doc/refman/8.0/en/flow-control-functions.html#operator_case) | Case operator |
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| [`IF()`](https://dev.mysql.com/doc/refman/8.0/en/flow-control-functions.html#function_if) | If/else construct |
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| [`IFNULL()`](https://dev.mysql.com/doc/refman/8.0/en/flow-control-functions.html#function_ifnull) | Null if/else construct |
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| [`NULLIF()`](https://dev.mysql.com/doc/refman/8.0/en/flow-control-functions.html#function_nullif) | Return NULL if expr1 = expr2 |
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| [`CASE`](#case) | Case operator |
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| [`IF()`](#if) | If/else construct |
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| [`IFNULL()`](#ifnull) | Null if/else construct |
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| [`NULLIF()`](#nullif) | Return `NULL` if expr1 = expr2 |
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## CASE
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The [`CASE`](https://dev.mysql.com/doc/refman/8.0/en/flow-control-functions.html#operator_case) operator enables you to perform conditional logic and customize query results based on specified conditions.
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Syntax:
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```sql
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CASE
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WHEN condition1 THEN result1
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WHEN condition2 THEN result2
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...
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ELSE default_result
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END
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```
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Example:
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```sql
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WITH RECURSIVE d AS (SELECT 1 AS n UNION ALL SELECT n+1 FROM d WHERE n<10)
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SELECT n, CASE WHEN n MOD 2 THEN "odd" ELSE "even" END FROM d;
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```
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```
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+----+----------------------------------------------+
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| n | CASE WHEN n MOD 2 THEN "odd" ELSE "even" END |
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+----+----------------------------------------------+
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| 1 | odd |
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| 2 | even |
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| 3 | odd |
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| 4 | even |
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| 5 | odd |
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| 6 | even |
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| 7 | odd |
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| 8 | even |
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| 9 | odd |
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| 10 | even |
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+----+----------------------------------------------+
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10 rows in set (0.00 sec)
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```
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## IF()
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The [`IF()`](https://dev.mysql.com/doc/refman/8.0/en/flow-control-functions.html#function_if) function enables you to perform different actions based on whether a value or expression is true or not.
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Syntax:
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```sql
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IF(condition, value_if_true, value_if_false)
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```
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Example:
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```sql
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WITH RECURSIVE d AS (SELECT 1 AS n UNION ALL SELECT n+1 FROM d WHERE n<10)
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SELECT n, IF(n MOD 2, "odd", "even") FROM d;
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```
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```
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+----+----------------------------+
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| n | IF(n MOD 2, "odd", "even") |
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+----+----------------------------+
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| 1 | odd |
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| 2 | even |
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| 3 | odd |
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| 4 | even |
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| 5 | odd |
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| 6 | even |
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| 7 | odd |
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| 8 | even |
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| 9 | odd |
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| 10 | even |
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+----+----------------------------+
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10 rows in set (0.00 sec)
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```
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## IFNULL()
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The [`IFNULL(expr1,expr2)`](https://dev.mysql.com/doc/refman/8.0/en/flow-control-functions.html#function_ifnull) function is used to handle NULL values in queries. If `expr1` is not `NULL`, it returns `expr1`; otherwise, it returns `expr2`.
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Example:
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```sql
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WITH data AS (SELECT NULL AS x UNION ALL SELECT 1 )
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SELECT x, IFNULL(x,'x has no value') FROM data;
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```
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```
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+------+----------------------------+
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| x | IFNULL(x,'x has no value') |
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+------+----------------------------+
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| NULL | x has no value |
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| 1 | 1 |
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+------+----------------------------+
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2 rows in set (0.0006 sec)
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```
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## NULLIF()
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The [`NULLIF(expr1,expr2)`](https://dev.mysql.com/doc/refman/8.0/en/flow-control-functions.html#function_nullif) function returns `NULL` if both arguments are the same or if the first argument is `NULL`. Otherwise, it returns the first argument.
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Example:
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```sql
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WITH RECURSIVE d AS (SELECT 1 AS n UNION ALL SELECT n+1 FROM d WHERE n<10)
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SELECT n, NULLIF(n+n, n+2) FROM d;
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```
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```
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+----+------------------+
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| n | NULLIF(n+n, n+2) |
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+----+------------------+
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| 1 | 2 |
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| 2 | NULL |
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| 3 | 6 |
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| 4 | 8 |
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| 5 | 10 |
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| 6 | 12 |
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| 7 | 14 |
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| 8 | 16 |
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| 9 | 18 |
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| 10 | 20 |
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+----+------------------+
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10 rows in set (0.00 sec)
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```
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In this example, when `n` equals `2`, both `n+n` and `n+2` equal `4`, making both arguments the same and causing the function to return `NULL`.

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