control-flow-functions: Update reference manual links (#16849) (#17321)

functions-and-operators/control-flow-functions.md

@@ -5,11 +5,138 @@ summary: Learn about the Control Flow functions.
 
 # Control Flow Functions
 
-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.
+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.
 
 | Name                                                                                            | Description                       |
 |:--------------------------------------------------------------------------------------------------|:----------------------------------|
-| [`CASE`](https://dev.mysql.com/doc/refman/8.0/en/flow-control-functions.html#operator_case)       | Case operator                     |
-| [`IF()`](https://dev.mysql.com/doc/refman/8.0/en/flow-control-functions.html#function_if)         | If/else construct                 |
-| [`IFNULL()`](https://dev.mysql.com/doc/refman/8.0/en/flow-control-functions.html#function_ifnull) | Null if/else construct            |
-| [`NULLIF()`](https://dev.mysql.com/doc/refman/8.0/en/flow-control-functions.html#function_nullif) | Return NULL if expr1 = expr2      |
+| [`CASE`](#case)       | Case operator                     |
+| [`IF()`](#if)         | If/else construct                 |
+| [`IFNULL()`](#ifnull) | Null if/else construct            |
+| [`NULLIF()`](#nullif) | Return `NULL` if expr1 = expr2      |
+
+## CASE
+
+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.
+
+Syntax:
+
+```sql
+CASE
+    WHEN condition1 THEN result1
+    WHEN condition2 THEN result2
+    ...
+    ELSE default_result
+END
+```
+
+Example:
+
+```sql
+WITH RECURSIVE d AS (SELECT 1 AS n UNION ALL SELECT n+1 FROM d WHERE n<10)
+SELECT n, CASE WHEN n MOD 2 THEN "odd" ELSE "even" END FROM d;
+```
+
+```
++----+----------------------------------------------+
+| n  | CASE WHEN n MOD 2 THEN "odd" ELSE "even" END |
++----+----------------------------------------------+
+|  1 | odd                                          |
+|  2 | even                                         |
+|  3 | odd                                          |
+|  4 | even                                         |
+|  5 | odd                                          |
+|  6 | even                                         |
+|  7 | odd                                          |
+|  8 | even                                         |
+|  9 | odd                                          |
+| 10 | even                                         |
++----+----------------------------------------------+
+10 rows in set (0.00 sec)
+```
+
+## IF()
+
+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.
+
+Syntax:
+
+```sql
+IF(condition, value_if_true, value_if_false)
+```
+
+Example:
+
+```sql
+WITH RECURSIVE d AS (SELECT 1 AS n UNION ALL SELECT n+1 FROM d WHERE n<10)
+SELECT n, IF(n MOD 2, "odd", "even") FROM d;
+```
+
+```
++----+----------------------------+
+| n  | IF(n MOD 2, "odd", "even") |
++----+----------------------------+
+|  1 | odd                        |
+|  2 | even                       |
+|  3 | odd                        |
+|  4 | even                       |
+|  5 | odd                        |
+|  6 | even                       |
+|  7 | odd                        |
+|  8 | even                       |
+|  9 | odd                        |
+| 10 | even                       |
++----+----------------------------+
+10 rows in set (0.00 sec)
+```
+
+## IFNULL()
+
+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`.
+
+Example:
+
+```sql
+WITH data AS (SELECT NULL AS x UNION ALL SELECT 1 )
+SELECT x, IFNULL(x,'x has no value') FROM data;
+```
+
+```
++------+----------------------------+
+| x    | IFNULL(x,'x has no value') |
++------+----------------------------+
+| NULL | x has no value             |
+|    1 | 1                          |
++------+----------------------------+
+2 rows in set (0.0006 sec)
+```
+
+## NULLIF()
+
+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.
+
+Example:
+
+```sql
+WITH RECURSIVE d AS (SELECT 1 AS n UNION ALL SELECT n+1 FROM d WHERE n<10)
+SELECT n, NULLIF(n+n, n+2) FROM d;
+```
+
+```
++----+------------------+
+| n  | NULLIF(n+n, n+2) |
++----+------------------+
+|  1 |                2 |
+|  2 |             NULL |
+|  3 |                6 |
+|  4 |                8 |
+|  5 |               10 |
+|  6 |               12 |
+|  7 |               14 |
+|  8 |               16 |
+|  9 |               18 |
+| 10 |               20 |
++----+------------------+
+10 rows in set (0.00 sec)
+```
+
+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`.