如何将键映射到MySQL选择查询中单个字段的值？

2024-05-15 04:52:03 228

您可以在MySQL中使用CASE语句将键映射到选择查询中单个字段的值。语法如下-

SELECT yourColumnName1,yourColumnName2,yourColumnName3,.........N
(
   CASE WHEN yourColumnName = 1 THEN 'ENABLED'
   ELSE 'DISABLED'
   END
) AS anyVariableName
FROM yourTableName;

您也可以IF()出于相同目的使用功能。语法如下-

SELECT yourColumnName1,yourColumnName2,yourColumnName3,.........N
,IF(yourColumnName,'ENABLED','DISABLED') as anyVariableName FROM yourTableName;

为了理解上述语法，让我们创建一个表。创建表的查询如下-

mysql> create table MapKeys
   -> (
   -> Id int NOT NULL AUTO_INCREMENT,
   -> Name varchar(20),
   -> isActive boolean,
   -> PRIMARY KEY(Id)
   -> );

使用insert命令在表中插入一些记录。查询如下-

mysql> insert into MapKeys(Name,isActive) values('Larry',true);

mysql> insert into MapKeys(Name,isActive) values('David',false);

mysql> insert into MapKeys(Name,isActive) values('Mike',true);

mysql> insert into MapKeys(Name,isActive) values('Carol',false);

mysql> insert into MapKeys(Name,isActive) values('Sam',false);

mysql> insert into MapKeys(Name,isActive) values('Bob',true);

使用select语句显示表中的所有记录。查询如下：

mysql> select *from MapKeys;

以下是输出-

+----+-------+----------+
| Id | Name  | isActive |
+----+-------+----------+
|  1 | Larry |        1 |
|  2 | David |        0 |
|  3 | Mike  |        1 |
|  4 | Carol |        0 |
|  5 | Sam   |        0 |
|  6 | Bob   |        1 |
+----+-------+----------+
6 rows in set (0.00 sec)

现在让我们使用case语句映射键。查询如下-

mysql> select Id,Name,
   -> (
   -> CASE WHEN isActive = 1 THEN 'ENABLED'
   -> ELSE 'DISABLED'
   -> END
   -> ) AS Status
   -> from MapKeys;

以下是输出-

+----+-------+----------+
| Id | Name  | Status   |
+----+-------+----------+
|  1 | Larry | ENABLED  |
|  2 | David | DISABLED |
|  3 | Mike  | ENABLED  |
|  4 | Carol | DISABLED |
| 5  | Sam   | DISABLED |
| 6  | Bob   | ENABLED  |
+----+-------+----------+
6 rows in set (0.00 sec)

您可以在IF()方法的帮助下实现相同-

mysql> select Id,Name,if(isActive,'ENABLED','DISABLED') as Status from MapKeys;

以下是输出-

+----+-------+----------+
| Id | Name  | Status   |
+----+-------+----------+
| 1 | Larry | ENABLED   |
| 2 | David | DISABLED |
| 3 | Mike   | ENABLED |
| 4 | Carol | DISABLED |
| 5 | Sam   | DISABLED |
| 6 | Bob   | ENABLED  |
+----+-------+----------+
6 rows in set (0.00 sec)

如何将键映射到MySQL选择查询中单个字段的值？

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