php+mysql 语句中被查询的字段可以设置变量么

Question

比如:

$condition = "a_size";
$single_size = 30;
$sql_count = "select count(*) as count from foot_info WHERE '$condition' = '$single_size' ";

我这样运行,貌似什么也查询不到
但很奇怪,如果我用字符串连接成sql语句就可以正常查询,如下

$sql_count = "select count(*) as count from foot_info WHERE".$condition." = '$single_size' ";

/////////////////////////////////////
我又输出了上面两个sql语句的输出,分别是

select count(*) as count from foot_info WHERE Foot_Size = '30'//可运行
select count(*) as count from foot_info WHERE 'Foot_Size' = '30'//不可运行

Answer 1

把$condition的引号去掉就好。。

Answer 2

mysql的查询字段不需要加引号或者select count(*) as count from foot_info WHERE `Foot_Size` = '30'//可运行

Answer 3

select * from foot_info WHERE 1=1;
select * from foot_info WHERE 1=0;

Answer 4

在postgresql中表名和字段名是单引号；在Mysql中表名和字段名必须``或不写
一句话概括就是Mysql的单引号和双引号会被Mysql当成字符串处理，`符号内是表名或者字段处理