53.7. Control Flow Functions

CASE

		
mysql> SELECT CASE 1 WHEN 1 THEN 'one'
    ->     WHEN 2 THEN 'two' ELSE 'more' END;
        -> 'one'
mysql> SELECT CASE WHEN 1>0 THEN 'true' ELSE 'false' END;
        -> 'true'
mysql> SELECT CASE BINARY 'B'
    ->     WHEN 'a' THEN 1 WHEN 'b' THEN 2 END;
        -> NULL		
		
		

IFNULL

		
mysql> SELECT IFNULL("TEST", 'OK');
+----------------------+
| IFNULL("TEST", 'OK') |
+----------------------+
| TEST                 |
+----------------------+
1 row in set (0.00 sec)

mysql> SELECT IFNULL(NULL, 'OK');
+--------------------+
| IFNULL(NULL, 'OK') |
+--------------------+
| OK                 |
+--------------------+
1 row in set (0.00 sec)		
		
		

NULLIF()

IF

		
mysql> SELECT IFNULL("TEST", 'OK');
+----------------------+
| IFNULL("TEST", 'OK') |
+----------------------+
| TEST                 |
+----------------------+
1 row in set (0.00 sec)

mysql> SELECT IFNULL(NULL, 'OK');
+--------------------+
| IFNULL(NULL, 'OK') |
+--------------------+
| OK                 |
+--------------------+
1 row in set (0.00 sec)		
		
		

原文出处：Netkiller 系列 手札
本文作者：陈景峯
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