1.union查询就是把2条或者多条sql语句的查询结果合并成一个结果集。如sql1: N行sql2: M行sql1 union sql2 --- NM行2.union满足什么条件就可以用了?只要结果集中的列数一致就可以.如都是2列或者N列3.如果Union后的结果有重复(即某2行,或N行,所有的列,值都一样),怎么办?这种情况是比较常见的,默认会去重.不想去重可以使用union all.4. 取自于2张表,通过别名让2个结果集的列一致。那么,如果取出的结果集,列名字不一样,还能否union.可以,而且取出的最终列名,以第1条sql为准 .参考实例代码: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 );1union会去掉重复的行:SELECT id,num FROM num_a UNION SELECT id, num FROM num_b2、UNION ALL不会过滤重复的行SELECT id,num FROM num_a UNION ALL SELECT id, num FROM num_b3、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 DESC4、把num_a和num_b不同的索引结果保留 相同的索引结果相加 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