Question

摘要

我正在寻找一个半连接（ish）查询，它可以选择多个客户并从其他表中加入他们最新的数据。

稍后，我希望直接追加条件到查询结尾：WHERE c.id IN (1,2,3)

问题

据我所知，我的要求排除了GROUP BY：

SELECT * FROM customer c
LEFT JOIN customer_address ca ON ca.customer_id = c.id
GROUP BY c.id
# PROBLEM: Cannot append conditions *after* GROUP BY!

对于大多数基于子查询的尝试，我的问题都是一样的。

作为额外的挑战，我不能严格使用半连接，因为我允许至少两种类型的电话号码（移动电话和固定电话），它们来自同一个表。因此，从电话表我可以加入每个客户的多个记录，即这不再是半连接。我目前的解决方案说明了这一点。

问题

底部的EXPLAIN结果看起来对我有效。我对么？每个子查询是否只执行一次？ 更新：对于外部查询中的每一行，似乎DEPENDENT SUBQUERY执行一次。如果我们能够避免这种情况，那就太好了。

对我正在做的事情有更好的解决方案吗？

的DDL

DROP TABLE IF EXISTS customer; CREATE TABLE `customer` ( `id` bigint(20) unsigned NOT NULL AUTO_INCREMENT, PRIMARY KEY (`id`) ); DROP TABLE IF EXISTS customer_address; CREATE TABLE `customer_address` ( `id` bigint(20) unsigned NOT NULL AUTO_INCREMENT, `customer_id` bigint(20) unsigned NOT NULL, `street` varchar(85) DEFAULT NULL, `house_number` int(10) unsigned DEFAULT NULL, PRIMARY KEY (`id`) ); DROP TABLE IF EXISTS customer_phone; CREATE TABLE `customer_phone` ( `id` bigint(20) unsigned NOT NULL AUTO_INCREMENT, `customer_id` bigint(20) unsigned NOT NULL, `phone` varchar(32) DEFAULT NULL, `type` tinyint(3) unsigned NOT NULL COMMENT '1=mobile,2=landline', PRIMARY KEY (`id`) ); insert ignore customer values (1); insert ignore customer_address values (1, 1, "OldStreet", 1),(2, 1, "NewStreet", 1); insert ignore customer_phone values (1, 1, "12345-M", 1),(2, 1, "12345-L-Old", 2),(3, 1, "12345-L-New", 2); SELECT * FROM customer; +----+ | id | +----+ | 1 | +----+ SELECT * FROM customer_address; +----+-------------+-----------+--------------+ | id | customer_id | street | house_number | +----+-------------+-----------+--------------+ | 1 | 1 | OldStreet | 1 | | 2 | 1 | NewStreet | 1 | +----+-------------+-----------+--------------+ SELECT * FROM customer_phone; +----+-------------+-------------+------+ | id | customer_id | phone | type | +----+-------------+-------------+------+ | 1 | 1 | 12345-M | 1 | | 2 | 1 | 12345-L-Old | 2 | | 3 | 1 | 12345-L-New | 2 | +----+-------------+-------------+------+
到目前为止
解决方案

SELECT * FROM customer c # Join the most recent address LEFT JOIN customer_address ca ON ca.id = (SELECT MAX(ca.id) FROM customer_address ca WHERE ca.customer_id = c.id) # Join the most recent mobile phone number LEFT JOIN customer_phone cphm ON cphm.id = (SELECT MAX(cphm.id) FROM customer_phone cphm WHERE cphm.customer_id = c.id AND cphm.`type` = 1) # Join the most recent landline phone number LEFT JOIN customer_phone cphl ON cphl.id = (SELECT MAX(cphl.id) FROM customer_phone cphl WHERE cphl.customer_id = c.id AND cphl.`type` = 2) # Yay conditions appended at the end WHERE c.id IN (1,2,3)

小提琴

这个小提琴使用给定的解决方案给出了合适的结果集。请参阅上面的问题。

http://sqlfiddle.com/#!9/98c57/3

Answer 1

我会避免使用那些依赖子查询，而是试试这个：

SELECT
      *
FROM customer c
      LEFT JOIN (
            SELECT
                  customer_id
                , MAX(id) AS currid
            FROM customer_phone
            WHERE type = 1
            GROUP BY
                  customer_id
      ) gm ON c.id = gm.customer_id
      LEFT JOIN customer_phone mobis ON gm.currid = mobis.id
      LEFT JOIN (
            SELECT
                  customer_id
                , MAX(id) AS currid
            FROM customer_phone
            WHERE type = 2
            GROUP BY
                  customer_id
      ) gl ON c.id = gl.customer_id
      LEFT JOIN customer_phone lands ON gl.currid = lands.id
WHERE c.id IN (1, 2, 3)
;

或者，或许：

SELECT
      *
FROM customer c
      LEFT JOIN (
            SELECT
                  customer_id
                , MAX(case when type = 1 then id end) AS mobid
                , MAX(case when type = 2 then id end) AS lndid
            FROM customer_phone
            GROUP BY
                  customer_id
      ) gp ON c.id = gp.customer_id
      LEFT JOIN customer_phone mobis ON gp.mobid = mobis.id
      LEFT JOIN customer_phone lands ON gp.lndid = lands.id
WHERE c.id IN (1, 2, 3)
;

请参阅：http://sqlfiddle.com/#!9/ef983/1/

MySQL复杂的半连接没有分组依据

1 个答案: