如何在MYSQl中合并具有两列共同的多个表答案

【问题标题】：How to merge multiple tables in MYSQl which have two columns in common如何在MYSQl中合并具有两列共同的多个表
【发布时间】：2015-01-23 07:12:06
【问题描述】：

我有四个表 table_1、table_2、table_3 和 table_4。他们四个都有这样的列：

table_1: age_grp, gender, height;
table_2: age_grp, gender, weight;
table_3:  age_grp, gender, shoesize;
table_4: age_group, gender, BMI;

我想创建一个包含列的新表：

age_grp, gender, height, weight, shoesize, BMI

我只想合并那些在所有表中age_grp and gender 相同的列。知道怎么做吗？

【问题讨论】：

标签： mysql sql

【解决方案1】：

这可以通过INNER JOIN轻松完成：

SELECT table_1.*, table_2.*, table_3.*, table_4.* FROM table_1  
  INNER JOIN table_2 ON table_1.age_grp = table_2.age_grp  
    AND table_1.gender = table_2.gender
  INNER JOIN table_3 ON table_2.age_grp = table_3.age_grp  
    AND table_2.gender = table_3.gender
  INNER JOIN table_4 ON table_3.age_grp = table_4.age_grp  
    AND table_3.gender = table_4.gender

如果您要求所有表中的所有数据在列中具有相同的值，您可以JOIN 任何表。

请注意，您不应在实际生产脚本中使用*，明确使用列名。

【讨论】：

第三个和第四个，我可以在逗号后面加名字吗？
@VMAtm 我有类似的查询，想优化 LINQ 查询
@ArijitMukherjee 在使用 LINQ 时，您应该问一个不同的问题。但是JOINs 不能轻易优化。
@VMAtm 不能去参加这个对话聊天吗？
@VMAtm 非常感谢。请不要介意我问这个，但我只有一个小问题。现在我不希望age_grp 和gender 出现多次。我的意思是现在新的选择有像age_grpgenderheightage_grpgenderweight等行。

【解决方案2】：

您很可能无法通过纯匹配获得您想要的结果。例如，以下将创建您描述的表：

insert into newtable 
    select t1.age_grp, t1.gender, t1.height, t2.weight, t3.shoesize, t4.BMI 
    from table_1 t1 
    inner join table_2 t2 on t1.age_grp = t2.age_grp 
        and t1.gender = t2.gender
    inner join table_3 t3 on t1.age_grp = t3.age_grp 
        and t1.gender = t3.gender
    inner join table_4 t4 on t1.age_grp = t4.age_grp 
        and t1.gender = t4.gender;

问题是，如果有 ANY 项失败，您将得不到一行。您可以考虑改用外连接。

【讨论】：

【解决方案3】：

尽管已经回答了这个请求，但我想为缺失值的情况添加一个答案，例如只为 age_grp/gender 对提供鞋码。

对于连接解决方案，您需要 MySQL 不支持的 FULL OUTER JOIN。使用 LEFT 和/或 RIGHT OUTER JOIN 来模仿这一点对于多个表来说可能会很痛苦。

这是一个使用 UNION ALL 和最终聚合的解决方案。

create table mytable as
select age_grp, gender, max(height) as height, max(weight) as weight, max(shoesize) as shoesize, max(bmi) as bmi
from
(
  select age_grp, gender, height, cast(null as unsigned integer) as weight, cast(null as unsigned integer) as shoesize, cast(null as unsigned integer) as bmi from table_1
  union all
  select age_grp, gender, cast(null as unsigned integer) as height, weight, cast(null as unsigned integer) as shoesize, cast(null as unsigned integer) as bmi from table_2
  union all
  select age_grp, gender, cast(null as unsigned integer) as height, cast(null as unsigned integer) as weight, shoesize, cast(null as unsigned integer) as bmi from table_3
  union all
  select age_group, gender, cast(null as unsigned integer) as height, cast(null as unsigned integer) as weight, cast(null as unsigned integer) as shoesize, bmi from table_4
) x
group by age_grp, gender;

我很惊讶CAST(NULL AS INT) 会导致语法错误，顺便说一句。我不得不把它改成CAST(NULL AS UNSIGNED INTEGER)。

SQL 小提琴：http://www.sqlfiddle.com/#!2/f4fa5c/1.

【讨论】：