在SQL中使用where子句选择多个列

Question

一个学生学习超过1个可选科目。我创建了两个名为optional_sub1和optional_sub2的列。我想使用sql语句从两列中仅选择一列。 SQL语句如下：

$sql= "SELECT * 
       FROM students 
       WHERE class = '$SelectClass' 
       && optional_sub1 = '$SelectSubject' 
       || optional_sub2 = '$SelectSubject'";

请修改以上语法。 谢谢。

Answer 1

分别为AND/OR运算符使用&&/||

$sql= "SELECT * FROM students WHERE class = '$SelectClass' 
and (optional_sub1 = '$SelectSubject' or optional_sub2 = '$SelectSubject')"

Answer 2

使用AND / OR代替C＃/ Java运算符。

$sql= "SELECT * FROM students 
       WHERE class = '$SelectClass' 
             and (optional_sub1 = '$SelectSubject' 
                  or 
                  optional_sub2 = '$SelectSubject'");

Answer 3

SELECT * 
FROM students 
WHERE class = '$SelectClass' 
and optional_sub1 = '$SelectSubject' 
and optional_sub2 = '$SelectSubject';