高效的SQL表联接或POSTGRES中的查询?怎么做和怎么做?

本文介绍了高效的SQL表联接或POSTGRES中的查询?怎么做和怎么做?的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！ 问题描述

我有下表

表1

ID1 YEAR1 1 1980 2 1964 3 1910

表2

ID2 YEAR2 VALUE 1 2000 A 1 1900 B 2 1950 C 2 1900 B 3 2000 C 3 1970 B 4 1900 D 4 1800 E

我想将这些表联接/查询到:

I would like to join / query these tables to:

对于表1中的每个ID1匹配项，添加一个名为VALUE的列-这样列名将为A，B，...，依此类推，并且该列将为TRUE(T)或FALSE(F).

for each ID1 in table 1 match, add a column named VALUE - so the column names would be A, B, ... and so on, and the column would be either TRUE(T) or FALSE(F).

如果存在ID1匹配表2中的ID2，并且表2中给定行的YEAR2匹配ID2小于表1中给定1的YEAR1，则A ...列为TRUE来自表2中ID2的行中具有A

The column A ... would be TRUE, if there was ID1 matching ID2 from table 2, and the YEAR2 for that given row in table 2 which matched ID2 was less than YEAR1 from that table 1 having given 1, and VALUE from row having ID2 from table 2 had A

因此，结果表如下:

ID1 YEAR1 A B C D E 1 1980 F T F F F 2 1964 F T T F F 3 1910 F F F F F

推荐答案

您可以使用如下查询:

SELECT t1.ID1, t1.YEAR1, COUNT(CASE WHEN t2.VALUE = 'A' AND t1.YEAR1 > t2.YEAR2 THEN 1 END) AS A, COUNT(CASE WHEN t2.VALUE = 'B' AND t1.YEAR1 > t2.YEAR2 THEN 1 END) AS B, COUNT(CASE WHEN t2.VALUE = 'C' AND t1.YEAR1 > t2.YEAR2 THEN 1 END) AS C, COUNT(CASE WHEN t2.VALUE = 'D' AND t1.YEAR1 > t2.YEAR2 THEN 1 END) AS D, COUNT(CASE WHEN t2.VALUE = 'E' AND t1.YEAR1 > t2.YEAR2 THEN 1 END) AS E, COUNT(CASE WHEN t2.VALUE = 'F' AND t1.YEAR1 > t2.YEAR2 THEN 1 END) AS F FROM Table1 AS t1 LEFT JOIN Table2 AS t2 ON t1.ID1 = t2.ID2 GROUP BY t1.ID1, t1.YEAR1

例如列A的值是1，则将其视为T，否则将其视为F.

If, e.g. value of column A is 1, then this is considered as T, otherwise it is considered as a F.

> 此处演示