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要添加查询中不存在的列？

2024-05-09 12:08:05 350

借助AS关键字，添加查询中不存在的列。语法如下-

SELECT yourColumnName1,yourColumnName2,....N,yourValue AS yourColumnName,....N' 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 ColumnDoesNotExists
    -> (
    -> UserId int,
    -> UserName varchar(20)
    -> );

示例

使用insert命令在表中插入一些记录。查询如下-

mysql> insert into ColumnDoesNotExists(UserId,UserName) values(100,'Larry');
mysql> insert into ColumnDoesNotExists(UserId,UserName) values(101,'Sam');
mysql> insert into ColumnDoesNotExists(UserId,UserName) values(102,'Mike');
mysql> insert into ColumnDoesNotExists(UserId,UserName) values(103,'David');
mysql> insert into ColumnDoesNotExists(UserId,UserName) values(104,'Robert');
mysql> insert into ColumnDoesNotExists(UserId,UserName) values(105,'Maxwell');
mysql> insert into ColumnDoesNotExists(UserId,UserName) values(106,'Bob');
mysql> insert into ColumnDoesNotExists(UserId,UserName) values(107,'John');
mysql> insert into ColumnDoesNotExists(UserId,UserName) values(108,'James');

使用select语句显示表中的所有记录。查询如下-

mysql> select *from ColumnDoesNotExists;

输出结果

+--------+----------+
| UserId | UserName |
+--------+----------+
|    100 |    Larry |
|    101 |      Sam |
|    102 |     Mike |
|    103 |    David |
|    104 |   Robert |
|    105 |  Maxwell |
|    106 |      Bob |
|    107 |     John |
|    108 |    James |
+--------+----------+
9 rows in set (0.00 sec)

示例

这是查询以添加查询中不存在的列名-

mysql> select UserId,UserName,23 AS Age from ColumnDoesNotExists;

输出结果

+--------+----------+-----+
| UserId | UserName | Age |
+--------+----------+-----+
|    100 |    Larry |  23 |
|    101 |      Sam |  23 |
|    102 |     Mike |  23 |
|    103 |    David |  23 |
|    104 |   Robert |  23 |
|    105 |  Maxwell |  23 |
|    106 |      Bob |  23 |
|    107 |     John |  23 |
|    108 |    James |  23 |
+--------+----------+-----+
9 rows in set (0.00 sec)

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要添加查询中不存在的列？

示例

示例

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