php - 仅选择联接表中的特定字段

Question

是否可以在左连接表中指定您想要的字段

IE

SELECT * FROM students
LEFT OUTER JOIN classes_enrolled (can i specify fields here)
ON students.student_id = classes_enrolled.student_id

我只想从左连接表classes_enrolled中获取字段主题，而不是附加匹配行中的所有字段

Answer 1

为什么不尝试类似的东西

SELECT  students.* ,
        classes_enrolled.subject 
FROM    students LEFT OUTER JOIN 
        classes_enrolled  ON students.student_id = classes_enrolled.student_id

实际上，指定字段名称而不是使用SELECT *

Answer 2

您可以选择以下任何一项：

使用子查询在指定列上进行选择，

SELECT  * 
FROM    students
        LEFT OUTER JOIN 
        (
            SELECT  student_id, subject 
            FROM    classes_enrolled
        ) b ON students.student_id = b.student_id

或者我更喜欢的——手动选择这些列，

SELECT  students.*,
        classes_enrolled.subject 
FROM    students
        LEFT OUTER JOIN classes_enrolled 
            ON students.student_id = classes_enrolled.student_id

Answer 3

试试下面的 SELECT , classes_enrolled.subject, * FROM students, LEFT OUTER JOIN classes_enrolled ON students.student_id = classes_enrolled.student_id