如何跨两个不相关的表进行 SUM? [英] How to do a SUM across two unrelated tables?

问题描述

我试图用 postgres 对两个不相关的表进行总结.使用 MySQL，我会做这样的事情:

SELECT SUM(table1.col1) AS sum_1, SUM(table2.col1) AS sum_2 FROM table1, table2

这应该给我一个包含两列名为 sum_1 和 sum_2 的表格.但是，postgres 没有给我这个查询的任何结果.

有什么想法吗?

解决方案

SELECT (SELECT SUM(table1.col1) FROM table1) AS sum_1,(SELECT SUM(table2.col1) FROM table2) AS sum_2;

你也可以写成:

SELECT t1.sum_c1, t1.sum_c2, t2.sum_t2_c1从(SELECT SUM(col1) sum_c1,SUM(col2) sum_c2从表 1) t1全外连接(SELECT SUM(col1) sum_t2_c1从表 2) t2 ON 1=1;

FULL JOIN 与无用条件一起使用，以便任何一个子查询都不会产生结果(空)，而不会导致更大的查询没有结果.

我不认为您编写的查询会产生您希望得到的结果，因为它在 table1 和 table2 之间进行了 CROSS JOIN，这会使每个 SUM 膨胀另一个表中的行数.请注意，如果 table1/table2 为空，则 CROSS JOIN 将导致 X 行乘 0 行返回空结果.

看看这个 SQL Fiddle 并比较结果.>

I'm trying to sum on two unrelated tables with postgres. With MySQL, I would do something like this :

SELECT SUM(table1.col1) AS sum_1, SUM(table2.col1) AS sum_2 FROM table1, table2

This should give me a table with two column named sum_1 and sum_2. However, postgres doesn't give me any result for this query.

Any ideas?

解决方案

SELECT (SELECT SUM(table1.col1) FROM table1) AS sum_1,
       (SELECT SUM(table2.col1) FROM table2) AS sum_2;

You can also write it as:

SELECT t1.sum_c1, t1.sum_c2, t2.sum_t2_c1
FROM
(
     SELECT SUM(col1) sum_c1,
            SUM(col2) sum_c2
 FROM table1
) t1
FULL OUTER JOIN
(
     SELECT SUM(col1) sum_t2_c1
     FROM table2
) t2 ON 1=1;

The FULL JOIN is used with a dud condition so that either subquery could produce no results (empty) without causing the greater query to have no result.

I don't think the query as you have written would have produced the result you expected to get, because it's doing a CROSS JOIN between table1 and table2, which would inflate each SUM by the count of rows in the other table. Note that if either table1/table2 is empty, the CROSS JOIN will cause X rows by 0 rows to return an empty result.

Look at this SQL Fiddle and compare the results.

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