SELECT *是否比MySQL中列出的40列快？
SELECT*比40列列表慢。在使用SELECT查询时列出列名是一个更好的选择。让我们看一个简单的例子并创建一个表-
create table DemoTable( Id int, Name varchar(20), Age int, ZipCode varchar(20), CountryName varchar(20) );
使用插入命令在表中插入一些记录-
insert into DemoTable values(101,'Chris',23,'0909332','US'); insert into DemoTable values(102,'Bob',24,'8747443','AUS'); insert into DemoTable values(103,'Adam',21,'9988833','UK');
使用select语句显示表中的所有记录-
select *from DemoTable;
这将产生以下输出-
+------+-------+------+---------+-------------+ | Id | Name | Age | ZipCode | CountryName | +------+-------+------+---------+-------------+ | 101 | Chris | 23 | 0909332 | US | | 102 | Bob | 24 | 8747443 | AUS | | 103 | Adam | 21 | 9988833 | UK | +------+-------+------+---------+-------------+ 3 rows in set (0.00 sec)
现在，我们将在使用MySQLSELECT时列出每个列名，因为它被认为是一种更快的方法-
select Id,Name,Age,ZipCode,CountryName from DemoTable;
这将产生以下输出-
+------+-------+------+---------+-------------+ | Id | Name | Age | ZipCode | CountryName | +------+-------+------+---------+-------------+ | 101 | Chris | 23 | 0909332 | US | | 102 | Bob | 24 | 8747443 | AUS | | 103 | Adam | 21 | 9988833 | UK | +------+-------+------+---------+-------------+ 3 rows in set (0.00 sec)