mysql的左联结与右连接

左连接where只影向右表，右连接where只影响左表。

左连接后的检索结果是显示tbl1的所有数据和tbl2中满足where
条件的数据。

左连接

SELECT

test1.id AS id,
test1.name1 AS NAME,
test2.id AS id2,
test2.name2 AS name1

FROM
test1

left JOIN test2 ON test1.id = test2.id

;

右连接

SELECT
test1.id AS id,
test1.name1 AS NAME,
test2.id AS id2,
test2.name2 AS name1

FROM
test1

right JOIN test2 ON test1.id = test2.id

;

全连接

SELECT
test1.id AS id,
test1.name1 AS NAME,
test2.id AS id2,
test2.name2 AS name1

FROM
test1

left JOIN test2 ON test1.id = test2.id

union

SELECT
test1.id AS id,
test1.name1 AS NAME,
test2.id AS id2,
test2.name2 AS name1

FROM
test1

right JOIN test2 ON test1.id = test2.id

;

选出test1中有test2中没有的

SELECT
test1.id AS id,
test1.name1 AS NAME,
test2.id AS id2,
test2.name2 AS name1

FROM
test1

left JOIN test2 ON test1.id = test2.id

WHERE test2.id is NULL

;