Condition judgment function of MySQL function summary

Condition judgment function is also called control flow function , According to different conditions , Process of executing response .mysql The functions for conditional judgment in if、ifunll and case etc. .

IF(expr,v1,v2) function

IF(expr,v1,v2)： If the expression expr yes TRUE（expr <> 0 and expr <> null）, The return value is V1; Otherwise, the return value is V2.

mysql> select if (1>2, 2, 3), if (1<2, 'yes', 'no');
+----------------+-----------------------+
| if (1>2, 2, 3) | if (1<2, 'yes', 'no') |
+----------------+-----------------------+
|              3 | yes                   |
+----------------+-----------------------+
1 row in set (0.00 sec)

mysql>

tip ：

If V1 perhaps V2 Only one of them is clear null, be if() The result type of the function is non null The result type of the expression .

IFNULL(v1,v2) function

ifnull(v1,v2)： If V1 Not for null, be ifnull() The return value of is v1; Otherwise, the return value is v2.

ifnull() The return value of is a number or string , The specific situation depends on the context .

mysql> select ifnull(1, 2), ifnull(null, 'yunweijia'), ifnull(1/0, 'heihei');
+--------------+---------------------------+-----------------------+
| ifnull(1, 2) | ifnull(null, 'yunweijia') | ifnull(1/0, 'heihei') |
+--------------+---------------------------+-----------------------+
|            1 | yunweijia                 | heihei                |
+--------------+---------------------------+-----------------------+
1 row in set (0.00 sec)

mysql>

CASE function

case expr when v1 then r1 [when v2 then 2]...[else rn+1]end： If expr The value is equal to some vn, Then return to the corresponding position then The latter result ; If it is not equal to all values , Then return to else hinder rn+1.

mysql> select case 2 when 1 then 'one' when '2' then 'two' else 'more' end;
+--------------------------------------------------------------+
| case 2 when 1 then 'one' when '2' then 'two' else 'more' end |
+--------------------------------------------------------------+
| two                                                          |
+--------------------------------------------------------------+
1 row in set (0.00 sec)

mysql>
mysql> select case 5 when 1 then 'one' when '2' then 'two' else 'more' end;
+--------------------------------------------------------------+
| case 5 when 1 then 'one' when '2' then 'two' else 'more' end |
+--------------------------------------------------------------+
| more                                                         |
+--------------------------------------------------------------+
1 row in set (0.00 sec)

mysql>

tip ：

May, in accordance with the shell Medium if Sentence to understand .

One case The default return value type of an expression is the compatible collection type of any return value , But it depends on the context .

thus , In this paper, the end .

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