MySQL - 有效地将两个select语句组合成一个结果和LIMIT

Question

对于约会应用程序,我有几个表,我需要查询单个输出,两个查询的LIMIT 10组合.此刻似乎很难做到,即使单独查询它们也不是问题,但LIMIT 10将无法​​工作,因为数字不准确(例如,不是LIMIT 5和LIMIT 5,一个查询可能返回0行,而另外10个,视情况而定).

members table
member_id | member_name
------------------------
     1         Herb
     2         Karen
     3         Megan

dating_requests
request_id | member1 | member2 | request_time
----------------------------------------------------
     1          1         2      2012-12-21 12:51:45

dating_alerts
alert_id | alerter_id | alertee_id | type | alert_time
-------------------------------------------------------
    5           3            2     platonic  2012-12-21 10:25:32

dating_alerts_status
status_id | alert_id | alertee_id | viewed | viewed_time
-----------------------------------------------------------
     4          5           2          0      0000-00-00 00:00:00 

想象一下你是凯伦并且刚刚登录,你应该看到这两个项目:

1. Herb requested a date with you.
2. Megan wants a platonic relationship with you.

在一个LIMIT为10的查询中.相反,这里有两个需要组合的查询:

1. Herb requested a date with you.
   -> query = "SELECT dr.request_id, dr.member1, dr.member2, m.member_name 
               FROM dating_requests dr 
               JOIN members m ON dr.member1=m.member_id 
               WHERE dr.member2=:loggedin_id 
               ORDER BY dr.request_time LIMIT 5";
2. Megan wants a platonic relationship with you.
   -> query = "SELECT da.alert_id, da.alerter_id, da.alertee_id, da.type,
                      da.alert_time, m.member_name
               FROM dating_alerts da
               JOIN dating_alerts_status das ON da.alert_id=das.alert_id
                    AND da.alertee_id=das.alertee_id
               JOIN members m ON da.alerter_id=m.member_id
               WHERE da.alertee_id=:loggedin_id AND da.type='platonic'
                     AND das.viewed='0' AND das.viewed_time<da.alert_time 
               ORDER BY da.alert_time LIMIT 5";

同样,有时两个表可能都是空的,或者1个表可能是空的,或者两个表都是空的(LIMIT 10可以启动)并按时间排序.有关如何有效执行此任务的查询的任何想法？欢迎提出想法,建议,编钟和优化.

Answer 1

您可以组合多个查询UNION,但前提是查询具有相同的列数.理想情况下,列不仅在数据类型中,而且在于它们的语义含义; 但是,MySQL并不关心语义,并且会通过强制转换为更通用的东西来处理不同的数据类型 - 因此,如果有必要,您可以重载列以使每个表具有不同的含义,然后确定在更高级代码中适当的含义(虽然我不建议这样做).

当列数不同时,或者想要从两个查询中实现更好/更少重载的数据对齐时,可以在SELECT语句中插入虚拟文字列.例如:

SELECT t.cola, t.colb, NULL, t.colc, NULL FROM t;

您甚至可以为第一个表保留一些列,为第二个表保留其他列,以便它们在NULL其他地方(但请记住,列名来自第一个查询,因此您可能希望确保它们全部在那里命名):

  SELECT a, b, c, d, NULL AS e, NULL AS f, NULL AS g FROM t1
UNION ALL -- specify ALL because default is DISTINCT, which is wasted here
  SELECT NULL, NULL, NULL, NULL, a, b, c FROM t2;

您可以尝试以这种方式对齐您的两个查询,然后将它们与UNION运算符组合; 通过应用LIMIT的UNION,你接近实现你的目标:

  (SELECT ...)
UNION
  (SELECT ...)
LIMIT 10;

剩下的唯一问题是,如上所述,来自第一个表的10个或更多记录将"推出"来自第二个表的任何记录.但是,我们可以利用ORDER BY外部查询来解决这个问题.

把它们放在一起:

(
  SELECT
    dr.request_time AS event_time, m.member_name,      -- shared columns
    dr.request_id, dr.member1, dr.member2,             -- request-only columns
    NULL AS alert_id, NULL AS alerter_id,              -- alert-only columns
      NULL AS alertee_id, NULL AS type
  FROM dating_requests dr JOIN members m ON dr.member1=m.member_id 
  WHERE dr.member2=:loggedin_id
  ORDER BY event_time LIMIT 10 -- save ourselves performing excessive UNION
) UNION ALL (
  SELECT
    da.alert_time AS event_time, m.member_name,        -- shared columns
    NULL, NULL, NULL,                                  -- request-only columns
    da.alert_id, da.alerter_id, da.alertee_id, da.type -- alert-only columns
  FROM
    dating_alerts da
    JOIN dating_alerts_status das USING (alert_id, alertee_id)
    JOIN members m ON da.alerter_id=m.member_id
  WHERE
    da.alertee_id=:loggedin_id
    AND da.type='platonic'
    AND das.viewed='0'
    AND das.viewed_time<da.alert_time
  ORDER BY event_time LIMIT 10 -- save ourselves performing excessive UNION
)
ORDER BY event_time
LIMIT 10;

当然,现在由您决定在读取结果集中的每条记录时确定要处理的行类型(建议您测试request_id和/或alert_id使用NULL值;或者可以在结果中添加一个额外的列,明确指出每个记录来自哪个表,但它应该是等效的,只要这些id列是NOT NULL).