control-flow-functions: Update reference manual links

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

@@ -6,11 +6,114 @@ aliases: ['/docs/dev/functions-and-operators/control-flow-functions/','/docs/dev
 
 # 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
+
+The `CASE` operator allows you to respond on multiple values and conditions.
+
+```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()` statement allows you to do something based on wheter a value or expression is true or not.
+
+```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()` functions returns the data of the expression if it is not NULL and if it is NULL it returns the data
+of the second argument of the function.
+
+```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()` function returns null if both arguments are the same. Otherwise it returns the first argument.
+
+```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)
+```
+
+Here n+n and n+2 both evaluate to 4 if n is 2, which makes both arguments the same and the function return NULL.