MySQL中表的幾種連線方式
阿新 • • 發佈:2020-11-23
MySQL表中的連線方式其實非常簡單,這裡就簡單的羅列出他們的特點。
表的連線(JOIN)可以分為內連線(JOIN/INNER JOIN)和外連線(LEFT JOIN/RIGHT JOIN)。
首先我們看一下我們本次演示的兩個表:
mysql> SELECT * FROM student; +------+----------+------+------+ | s_id | s_name | age | c_id | +------+----------+------+------+ | 1 | xiaoming | 13 | 1 | | 2 | xiaohong | 41 | 4 | | 3 | xiaoxia | 22 | 3 | | 4 | xiaogang | 32 | 1 | | 5 | xiaoli | 41 | 2 | | 6 | wangwu | 13 | 2 | | 7 | lisi | 22 | 3 | | 8 | zhangsan | 11 | 9 | +------+----------+------+------+ 8 rows in set (0.00 sec) mysql> SELECT * FROM class; +------+---------+-------+ | c_id | c_name | count | +------+---------+-------+ | 1 | MATH | 65 | | 2 | CHINESE | 70 | | 3 | ENGLISH | 50 | | 4 | HISTORY | 30 | | 5 | BIOLOGY | 40 | +------+---------+-------+ 5 rows in set (0.00 sec)
首先,表要能連線的前提就是兩個表中有相同的可以比較的列。
1.內連線
mysql> SELECT * FROM student INNER JOIN class ON student.c_id = class.c_id; +------+----------+------+------+------+---------+-------+ | s_id | s_name | age | c_id | c_id | c_name | count | +------+----------+------+------+------+---------+-------+ | 1 | xiaoming | 13 | 1 | 1 | MATH | 65 | | 2 | xiaohong | 41 | 4 | 4 | HISTORY | 30 | | 3 | xiaoxia | 22 | 3 | 3 | ENGLISH | 50 | | 4 | xiaogang | 32 | 1 | 1 | MATH | 65 | | 5 | xiaoli | 41 | 2 | 2 | CHINESE | 70 | | 6 | wangwu | 13 | 2 | 2 | CHINESE | 70 | | 7 | lisi | 22 | 3 | 3 | ENGLISH | 50 | +------+----------+------+------+------+---------+-------+ 7 rows in set (0.00 sec)
簡單的講,內連線就是把兩個表中符合條件的行的所有資料一起展示出來,即如果不符合條件,即在表A中找得到但是在B中沒有(或者相反)的資料不予以顯示。
2.外連線
mysql> SELECT * FROM student LEFT JOIN class ON student.c_id = class.c_id; +------+----------+------+------+------+---------+-------+ | s_id | s_name | age | c_id | c_id | c_name | count | +------+----------+------+------+------+---------+-------+ | 1 | xiaoming | 13 | 1 | 1 | MATH | 65 | | 2 | xiaohong | 41 | 4 | 4 | HISTORY | 30 | | 3 | xiaoxia | 22 | 3 | 3 | ENGLISH | 50 | | 4 | xiaogang | 32 | 1 | 1 | MATH | 65 | | 5 | xiaoli | 41 | 2 | 2 | CHINESE | 70 | | 6 | wangwu | 13 | 2 | 2 | CHINESE | 70 | | 7 | lisi | 22 | 3 | 3 | ENGLISH | 50 | | 8 | zhangsan | 11 | 9 | NULL | NULL | NULL | +------+----------+------+------+------+---------+-------+ 8 rows in set (0.00 sec) mysql> SELECT * FROM student RIGHT JOIN class ON student.c_id = class.c_id; +------+----------+------+------+------+---------+-------+ | s_id | s_name | age | c_id | c_id | c_name | count | +------+----------+------+------+------+---------+-------+ | 1 | xiaoming | 13 | 1 | 1 | MATH | 65 | | 4 | xiaogang | 32 | 1 | 1 | MATH | 65 | | 5 | xiaoli | 41 | 2 | 2 | CHINESE | 70 | | 6 | wangwu | 13 | 2 | 2 | CHINESE | 70 | | 3 | xiaoxia | 22 | 3 | 3 | ENGLISH | 50 | | 7 | lisi | 22 | 3 | 3 | ENGLISH | 50 | | 2 | xiaohong | 41 | 4 | 4 | HISTORY | 30 | | NULL | NULL | NULL | NULL | 5 | BIOLOGY | 40 | +------+----------+------+------+------+---------+-------+ 8 rows in set (0.00 sec)
上面分別展示了外連線的兩種情況:左連線和右連線。這兩種幾乎是一樣的,唯一的區別就是左連線的主表是左邊的表,右連線的主表是右邊的表。而外連線與內連線不同的地方就是它會將主表的所有行都予以顯示,而在主表中有,其他表中沒有的資料用NULL代替。
總結
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