一、mysql查詢與許可權

（一）資料庫關聯查詢

**內連接查詢（inner join）**

查詢兩個表共有的數據,交集

SELECT * FROM tb1 INNER JOIN tb2 ON 條件

所有有宿舍的學員

左表查詢(左關聯查詢)(left join)

查詢兩個表共有的數據,和左表所有的數據，左表有右表沒有的部分用null代替

SELECT * FROM tb1 LEFT JOIN tb2 ON 條件

 所有學員的住宿情況

右表查詢(右關聯查詢)(right join)

查詢兩個表共有的數據,和右表表所有的數據，右表有左表沒有的部分用null代替

SELECT * FROM tb1 RIGHT JOIN tb2 ON 條件

所有宿舍的學員情況

沒有學員的宿舍

沒有宿舍的學員

（二）多表(三表)關聯查詢

1、內關聯查詢
查看所有php的課程

select * from major inner join major_course inner join course where 
major.id = major_course.major_id and major_course.course_id = course.id 
and major.id= 1;

規範寫法：

SELECT
m.id AS m_id,
m.name AS m_name,
c.id AS c_id,
c.name AS c_name
FROM
major AS m
INNER JOIN major_course AS mc
INNER JOIN course AS c
WHERE
m.id = mc.major_id
AND mc.course_id = c.id;

2、關聯子查詢
查詢所有含有電腦基礎的專業

SELECT
name
FROM
major
WHERE
id IN (
SELECT
mc.major_id
FROM
course AS c
LEFT JOIN major_course AS mc ON c.id = mc.course_id
WHERE
c.id = 1
);