Question

php+mysql 语句中被查询的字段可以设置变量么

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

求大神解释

Answer 1

查询语句中可以直接放变量；

$abc = '内容';
$sql = "select * from table where a = '{$abc}'";
// 这个语句等同于: $sql = "select * from table where a = '内容'";
// 其次，字段名 和 字段值都可以用变量来代替，但是字段名不能加引号，否则会报 mysql  语法错误；

Answer 2

字段名是不要加引号的