其他
SQL中 left join 左表合并去重实用技巧
CREATE TABLE `table1` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`name` varchar(60) DEFAULT NULL,
`age` varchar(200) DEFAULT NULL,
`sponsor_id` varchar(20) DEFAULT NULL COMMENT '业务发起人',
`gmt_create_user` int(11) NOT NULL COMMENT '创建人id',
`gmt_create` datetime NOT NULL DEFAULT CURRENT_TIMESTAMP COMMENT '创建时间',
`gmt_modified` datetime DEFAULT NULL COMMENT '修改时间',
`gmt_modified_user` int(11) DEFAULT NULL COMMENT '修改人id',
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=12 DEFAULT CHARSET=utf8mb4 COMMENT='测试表1';
CREATE TABLE `table2` (
`kid` int(11) NOT NULL AUTO_INCREMENT,
`name` varchar(60) DEFAULT NULL,
`sponsor_id` varchar(20) DEFAULT NULL COMMENT '业务发起人',
`type` int(11) NOT NULL COMMENT '创建人id',
`gmt_create` datetime NOT NULL DEFAULT CURRENT_TIMESTAMP COMMENT '创建时间',
`gmt_modified` datetime DEFAULT NULL COMMENT '修改时间',
`gmt_modified_user` int(11) DEFAULT NULL COMMENT '修改人id',
PRIMARY KEY (`kid`)
) ENGINE=InnoDB AUTO_INCREMENT=12 DEFAULT CHARSET=utf8mb4 COMMENT='测试表2';
INSERT INTO `table1`(`id`, `name`, `age`, `sponsor_id`, `gmt_create_user`, `gmt_create`, `gmt_modified`, `gmt_modified_user`) VALUES (1, 't1', '11', '10', 1, '2018-10-10 20:34:03', NULL, NULL);
INSERT INTO `table1`(`id`, `name`, `age`, `sponsor_id`, `gmt_create_user`, `gmt_create`, `gmt_modified`, `gmt_modified_user`) VALUES (2, 't2', '12', '10', 2, '2018-10-10 20:34:03', NULL, NULL);
INSERT INTO `table1`(`id`, `name`, `age`, `sponsor_id`, `gmt_create_user`, `gmt_create`, `gmt_modified`, `gmt_modified_user`) VALUES (3, 't3', '13', '10', 3, '2018-10-10 20:34:03', NULL, NULL);
INSERT INTO `table1`(`id`, `name`, `age`, `sponsor_id`, `gmt_create_user`, `gmt_create`, `gmt_modified`, `gmt_modified_user`) VALUES (4, 't4', '14', '20', 4, '2018-10-10 20:34:03', NULL, NULL);
INSERT INTO `table2`(`kid`, `name`, `sponsor_id`, `type`, `gmt_create`, `gmt_modified`, `gmt_modified_user`) VALUES (1, 't1', '10', 1, '2018-10-10 20:38:10', NULL, NULL);
INSERT INTO `table2`(`kid`, `name`, `sponsor_id`, `type`, `gmt_create`, `gmt_modified`, `gmt_modified_user`) VALUES (2, 't2', '10', 1, '2018-10-10 20:38:10', NULL, NULL);
INSERT INTO `table2`(`kid`, `name`, `sponsor_id`, `type`, `gmt_create`, `gmt_modified`, `gmt_modified_user`) VALUES (3, 't3', '10', 1, '2018-10-10 20:38:10', NULL, NULL);
INSERT INTO `table2`(`kid`, `name`, `sponsor_id`, `type`, `gmt_create`, `gmt_modified`, `gmt_modified_user`) VALUES (4, 't4', '10', 1, '2018-10-10 20:38:10', NULL, NULL);
INSERT INTO `table2`(`kid`, `name`, `sponsor_id`, `type`, `gmt_create`, `gmt_modified`, `gmt_modified_user`) VALUES (5, 't5', '10', 1, '2018-10-10 20:38:10', NULL, NULL);
INSERT INTO `table2`(`kid`, `name`, `sponsor_id`, `type`, `gmt_create`, `gmt_modified`, `gmt_modified_user`) VALUES (6, 't6', '10', 1, '2018-10-10 20:38:10', NULL, NULL);
INSERT INTO `table2`(`kid`, `name`, `sponsor_id`, `type`, `gmt_create`, `gmt_modified`, `gmt_modified_user`) VALUES (7, 't7', '10', 2, '2018-10-10 20:38:10', NULL, NULL);
INSERT INTO `table2`(`kid`, `name`, `sponsor_id`, `type`, `gmt_create`, `gmt_modified`, `gmt_modified_user`) VALUES (8, 't1', '11', 1, '2018-10-10 20:38:10', NULL, NULL);
SELECT
a.*,
b.type
FROM
table1 a
LEFT JOIN table2 b ON a.sponsor_id = b.sponsor_id
WHERE
b.type = 1
AND a.sponsor_id = 10;
解决方法:
使用非唯一标识的字段做关联
select DISTINCT(id) from a left join b on a.id=b.aid
SELECT
DISTINCT(a.id), a.*,
b.type
FROM
table1 a
LEFT JOIN table2 b ON a.sponsor_id = b.sponsor_id
WHERE
b.type = 1
AND a.sponsor_id = 10;
SELECT
DISTINCT a.*,
b.type
FROM
table1 a
LEFT JOIN table2 b ON a.sponsor_id = b.sponsor_id
WHERE
b.type = 1
AND a.sponsor_id = 10;
select * from a left join(select id from b group by id) as b on a.id=b.aid
SELECT
a.*,
b.type
FROM
table1 a
LEFT JOIN ( SELECT * FROM table2 GROUP BY sponsor_id ) AS b ON a.sponsor_id = b.sponsor_id
WHERE
b.type = 1
AND a.sponsor_id = 10;
select * from a left join (select max(id) from table group by id) as b on a.id=b.aid
SELECT
a.*,
b.type
FROM
table1 a
LEFT JOIN ( SELECT MAX( kid ), type, sponsor_id FROM table2 GROUP BY sponsor_id ) AS b ON a.sponsor_id = b.sponsor_id
WHERE
b.type = 1
AND a.sponsor_id = 10;
SELECT
a.*
FROM
table1 a
WHERE
a.sponsor_id IN ( SELECT sponsor_id FROM table2 WHERE type = 1 AND sponsor_id = 10 );
SELECT
a.*,
1
FROM
table1 a
WHERE
a.sponsor_id IN ( SELECT sponsor_id FROM table2 WHERE type = 1 AND sponsor_id = 10 );
SELECT A.卡号,B.手机号码
FROM A
LEFT JOIN B
ON A.客户号=B.客户号
WHERE B.联系方式='2'
SELECT A.卡号,B.手机号码
FROM A
LEFT JOIN (
SELECT * FROM B
B.联系方式='2'
)B
ON A.客户号=B.客户号
表结构 Left Join Right Join Inner Join 表的关联修改和删除 笛卡尔积
1、表结构
2、Left Join
Select * From A left join B on A.aid = B.bid;
A表所有记录都会显示,A表中没有被匹配的行(如aid=5、6的行)相应内容则为NULL。 返回的记录数一定大于A表的记录数,如A表中aid=7行被B表匹配了3次(因为B表有三行bid=7)。
注意:在Access中A.aid、B.bid不能缩写成aid、bid,否则会提示“不支持链接表达式”,这一点不同于Where查询。
3、Right Join
4、Inner Join
Select * From A inner join B on A.aid = B.bid;
Select * From A, B Where A.aid = B.bid
5、表的关联修改和删除
5.1修改
update A left join B on A.aid = B.bidset A.aname = B.bname
aid=5、6的记录,被更新为NULL aid=7的记录,被更新了3次,依次是“b1997-1”、“b1997-2”、“b1997-3”,因此其结果为最后一次更新“b1997-3”
update A left join B on A.aid = B.bidset A.aname = B.bname where A.aid <> 5
5.2删除
Delete From A inner join B on A.aid = B.bid where B.bname = "b1991"
Delete From A Where A.aid In (Select bid From B Where B.bname="b1991")
6、笛卡尔积
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