如何在MySQL中连接多个表

我需要FULL OUTER JOIN多个表.我知道如何从here开始加入两个表.但我有几个表,我不能将它们应用于它们.我怎样才能实现它？
我的SQL代码,如下：

INSERT INTO table
(
  customer_id
 ,g01
 ,g02
 ,g03
 ,has_card
 ,activity
  )
  SELECT sgd.customer_id, sgd.g01,sgd.g02,sgd.g03,sc.value, a.activity
  FROM s_geo_data sgd
  LEFT JOIN s_category sc
  ON sc.customer_id = sgd.customer_id
    UNION
  SELECT sgd.customer_id, sgd.g01,sgd.g02,sgd.g03,sc.value, a.activity
  FROM s_geo_data sgd
  RIGHT JOIN s_category sc
  ON sc.customer_id = sgd.customer_id

    UNION

  SELECT sgd.customer_id, sgd.g01,sgd.g02,sgd.g03,sc.value, a.activity
  FROM s_geo_data sgd
  LEFT JOIN s_activity a
  ON a.customer_id = sgd.customer_id
    UNION
  SELECT sgd.customer_id, sgd.g01,sgd.g02,sgd.g03,sc.value, a.activity
  FROM s_geo_data sgd
  RIGHT JOIN s_activity a
  ON a.customer_id = sgd.customer_id

我也尝试了这个查询：

INSERT INTO reportls.table
(
  customer_id
 ,g01
 ,g02
 ,g03
 ,has_card
 ,activity
  )
  SELECT sgd.customer_id, sgd.g01,sgd.g02,sgd.g03,sc.value, a.activity
  FROM s_geo_data sgd
  LEFT JOIN s_category sc
  ON sc.customer_id = sgd.customer_id
  LEFT JOIN s_activity a
  ON sc.customer_id = sgd.customer_id

    UNION

  SELECT sgd.customer_id, sgd.g01,sgd.g02,sgd.g03,sc.value, a.activity
  FROM s_geo_data sgd
  LEFT JOIN s_category sc
  ON sc.customer_id = sgd.customer_id
  RIGHT JOIN s_activity a
  ON a.customer_id = sgd.customer_id

    UNION

  SELECT sgd.customer_id, sgd.g01,sgd.g02,sgd.g03,sc.value, a.activity
  FROM s_geo_data sgd
  RIGHT JOIN s_category sc
  ON sc.customer_id = sgd.customer_id
  LEFT JOIN s_activity a
  ON a.customer_id = sgd.customer_id

上次查询执行时间很长,我需要更快的查询.

解决方法:

我想在3个表上有一个FULL OUTER JOIN,你需要这样做：

SELECT t1.value, t2.value, t3.value
FROM t1 LEFT JOIN t2 ON t1.value = t2.value
        LEFT JOIN t3 ON t1.value = t3.value
UNION ALL
SELECT t1.value, t2.value, t3.value
FROM t2 LEFT JOIN t1 ON t1.value = t2.value
        LEFT JOIN t3 ON t2.value = t3.value
WHERE t1.value IS NULL
UNION ALL
SELECT t1.value, t2.value, t3.value
FROM t3 LEFT JOIN t1 ON t1.value = t3.value
        LEFT JOIN t2 ON t2.value = t3.value
WHERE t1.value IS NULL AND t2.value IS NULL

作为替代方案：

SELECT t1.value, t2.value, t3.value
FROM t1 FULL OUTER JOIN t2 ON t1.value = t2.value
        FULL OUTER JOIN t3 ON t1.value = t3.value

我建议你创建一些临时表,如t1,t2和t3,用于存储查询结果,然后使用上面的查询.

标签：mysql,join,outer-join,query-performance
来源： https://codeday.me/bug/20190702/1359869.html