MySQL-几种关联

左表t1：

DROP TABLE IF EXISTS t1;
CREATE TABLE t1 (id INT NOT NULL,NAME VARCHAR(20));
INSERT INTO t1 VALUES (1,'t1a');
INSERT INTO t1 VALUES (2,'t1b');
INSERT INTO t1 VALUES (3,'t1c');
INSERT INTO t1 VALUES (4,'t1d');
INSERT INTO t1 VALUES (5,'t1f');

右表 t2：

DROP TABLE IF EXISTS t1;
CREATE TABLE t2 (id INT NOT NULL,NAME VARCHAR(20));
INSERT INTO t2 VALUES (2,'t2b');
INSERT INTO t2 VALUES (3,'t2c');
INSERT INTO t2 VALUES (4,'t2d');
INSERT INTO t2 VALUES (5,'t2f');
INSERT INTO t2 VALUES (6,'t2a');

 图解：

#笛卡尔积：两表关联，把左表的列和右表的列通过笛卡尔积的形式表达出来。

SELECT * FROM t1 JOIN t2

#左连接：两表关联，左表全部保留，右表关联不上用null表示。

SELECT * FROM t1 LEFT JOIN t2 ON t1.id =t2.id

#右连接：右表全部保留，左表关联不上的用null表示。

SELECT * FROM t1 RIGHT JOIN t2 ON t1.id =t2.`id`

#内连接：两表关联，保留两表中交集的记录。

SELECT * FROM t1 INNER JOIN t2 ON t1.id=t2.`id`

#左独有：两表关联，查询左表独有的数据。

SELECT * FROM t1 LEFT JOIN t2 ON t1.id =t2.`id` WHERE t2.id IS NULL

#右独有：两表关联，查询右表独有的数据。

SELECT * FROM t1 RIGHT JOIN t2 ON t1.id =t2.id WHERE t1.id IS NULL

#全连接：两表关联，查询它们的所有记录

SELECT * FROM t1 RIGHT JOIN t2 ON t1.id =t2.`id`
UNION
SELECT * FROM t1 LEFT JOIN t2 ON t1.id =t2.id

#并集去交集：两表关联，取并集然后去交集。

SELECT * FROM t1 LEFT JOIN t2 ON t1.id =t2.`id` WHERE t2.id IS NULL
UNION
SELECT * FROM t1 RIGHT JOIN t2 ON t1.id =t2.id WHERE t1.id IS NULL