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法律学添加查询中不存在的列？
在查询中添加不存在的列，可以使用 AS 关键字。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) -> ); Query OK, 0 rows affected (0.67 sec)
示例
使用插入命令在表中插入一些记录。查询如下 −
mysql> insert into ColumnDoesNotExists(UserId,UserName) values(100,'Larry'); Query OK, 1 row affected (0.14 sec) mysql> insert into ColumnDoesNotExists(UserId,UserName) values(101,'Sam'); Query OK, 1 row affected (0.22 sec) mysql> insert into ColumnDoesNotExists(UserId,UserName) values(102,'Mike'); Query OK, 1 row affected (0.15 sec) mysql> insert into ColumnDoesNotExists(UserId,UserName) values(103,'David'); Query OK, 1 row affected (0.15 sec) mysql> insert into ColumnDoesNotExists(UserId,UserName) values(104,'Robert'); Query OK, 1 row affected (0.10 sec) mysql> insert into ColumnDoesNotExists(UserId,UserName) values(105,'Maxwell'); Query OK, 1 row affected (0.20 sec) mysql> insert into ColumnDoesNotExists(UserId,UserName) values(106,'Bob'); Query OK, 1 row affected (0.17 sec) mysql> insert into ColumnDoesNotExists(UserId,UserName) values(107,'John'); Query OK, 1 row affected (0.17 sec) mysql> insert into ColumnDoesNotExists(UserId,UserName) values(108,'James'); Query OK, 1 row affected (0.18 sec)
使用 select 语句显示表中的所有记录。查询如下 −
mysql> select *from ColumnDoesNotExists;
输出
+--------+----------+ | 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
以下查询可以添加查询中不存在的列名 −
mysql> select UserId,UserName,23 AS Age from ColumnDoesNotExists;
输出
+--------+----------+-----+ | 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)
更新于: 26-Jun-2020
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