三、UNION ALL不会过滤重复的行

mysql中的union用法以及子查询综合运用,mysqlunion

union查询正是把二条要么多条sql语句的询问结果,合并成三个结果集。

如:sql1: N行,sql2: M行,sql1 union sql2 —> N+M行

1、能无法从2张表查询再union呢?

可以,union 合并的是”结果集”,不区分在自于哪一张表.

贰、取自于二张表,通过”别名”让2个结实集的列壹致。那么,假使收取的结果集,列名字不平等,还是能还是不能够union.

能够,而且抽出的结尾列名,以第一条sql为准

三、union知足哪些条件就足以用了?

假定结果集中的列数1致就能够.(如都以二列依旧N列)

肆、union后结果集,可以还是不可以再排序呢?

可以的。Sql1 union sql2 order by 字段

瞩目: order by 是针对性合并后的结果集排的序.

五、假若Union后的结果有再一次(即某二行,或N行,全部的列,值都一样),怎么办?

这种场馆是相比较遍布的,私下认可会去重.

六、假诺不想去重怎么办?

union all

 

下边通过实例来验证union的常用本性以及采取:

CREATE TABLE num_a (
    id VARCHAR( 3 ) NOT NULL,
    num INT(3 ) UNSIGNED NOT NULL
)CHARSET utf8 ENGINE MYISAM;
CREATE TABLE num_b (
    id VARCHAR( 3 ) NOT NULL,
    num INT(3 ) UNSIGNED NOT NULL
)CHARSET utf8 ENGINE MYISAM;

INSERT INTO num_a VALUES ( 'a', 5 );
INSERT INTO num_a VALUES ( 'b', 10 );
INSERT INTO num_a VALUES ( 'c', 15 );
INSERT INTO num_a VALUES ( 'd', 10 );

INSERT INTO num_b VALUES ( 'b', 5 );
INSERT INTO num_b VALUES ( 'c', 15 );
INSERT INTO num_b VALUES ( 'd', 20 );
INSERT INTO num_b VALUES ( 'e', 99 );

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 一,union会去掉重复的行

SELECT id,num FROM num_a UNION SELECT id, num FROM num_b

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二、order by对union后的结果集排序

SELECT id,num FROM num_a UNION SELECT id, num FROM num_b ORDER BY num DESC

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三、UNION ALL不会过滤重复的行

SELECT id,num FROM num_a UNION ALL SELECT id, num FROM num_b

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4、把num_a和num_b差别的目录结果保留, 同样的目录结果相加 
然后输出:

SELECT a.id, ( a.num + b.num ) AS num FROM num_a AS a INNER JOIN num_b AS b ON a.id = b.id
UNION ALL
SELECT * FROM num_a AS a WHERE NOT EXISTS( SELECT * FROM num_b AS b WHERE a.id = b.id )
UNION ALL
SELECT * FROM num_b AS b WHERE NOT EXISTS( SELECT * FROM num_a AS a WHERE a.id = b.id )
ORDER BY id ASC

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5、第3种方法用子查询分组总括,也得以到达同等的成效

SELECT id, SUM( num ) AS num FROM ( SELECT * FROM num_a a UNION ALL SELECT * FROM num_b b ) tmp
GROUP BY id;

 

union查询正是把二条要么多条sql语句的查询结果,合并成三个结果集。
如:sql一: N行,sql二: M行…

3、union满意哪些规范就足以用了?

4、union后结果集,可以还是不可以再排序呢?

SELECT id,num FROM num_a UNION ALL SELECT id, num FROM num_b

 

CREATE TABLE num_a (
    id VARCHAR( 3 ) NOT NULL,
    num INT(3 ) UNSIGNED NOT NULL
)CHARSET utf8 ENGINE MYISAM;
CREATE TABLE num_b (
    id VARCHAR( 3 ) NOT NULL,
    num INT(3 ) UNSIGNED NOT NULL
)CHARSET utf8 ENGINE MYISAM;

INSERT INTO num_a VALUES ( 'a', 5 );
INSERT INTO num_a VALUES ( 'b', 10 );
INSERT INTO num_a VALUES ( 'c', 15 );
INSERT INTO num_a VALUES ( 'd', 10 );

INSERT INTO num_b VALUES ( 'b', 5 );
INSERT INTO num_b VALUES ( 'c', 15 );
INSERT INTO num_b VALUES ( 'd', 20 );
INSERT INTO num_b VALUES ( 'e', 99 );

 

SELECT a.id, ( a.num + b.num ) AS num FROM num_a AS a INNER JOIN num_b AS b ON a.id = b.id
UNION ALL
SELECT * FROM num_a AS a WHERE NOT EXISTS( SELECT * FROM num_b AS b WHERE a.id = b.id )
UNION ALL
SELECT * FROM num_b AS b WHERE NOT EXISTS( SELECT * FROM num_a AS a WHERE a.id = b.id )
ORDER BY id ASC

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 一,union会去掉重复的行

 

union查询正是把二条照旧多条sql语句的询问结果,合并成一个结出集。

可以的。Sql1 union sql2 order by 字段

一、能或无法从贰张表查询再union呢?

  

六、如若不想去重怎么办?

上边通超过实际例来验证union的常用个性以及使用:

这种气象是相比较遍布的,暗中认可会去重.

 

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