SQL 在 PostgreSQL 中合并两个 SELECT 查询
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Combine two SELECT queries in PostgreSQL
提问by Bwyss
I would like to combine two select queries with UNION.
How can I use the result from the first SELECTin the second SELECT?
我想将两个选择查询与UNION.
如何SELECT在第二个中使用第一个的结果SELECT？
(SELECT carto_id_key FROM table1
WHERE tag_id = 16)
UNION
(SELECT * FROM table2
WHERE carto_id_key = <the carto_id result from above> )
回答by Erwin Brandstetter
Use a CTEto reuse the result from a subquery in more than one SELECT.
You need PostgreSQL 8.4+ for that:
使用CTE在多个SELECT.
为此，您需要 PostgreSQL 8.4+：
WITH x AS (SELECT carto_id_key FROM table1 WHERE tag_id = 16)
SELECT carto_id_key
FROM x
UNION ALL
SELECT t2.some_other_id_key
FROM x
JOIN table2 t2 ON t2.carto_id_key = x.carto_id_key
You most probably want UNION ALLinstead of UNION. Doesn't exclude duplicates and is faster this way.
你很可能想要UNION ALL而不是UNION. 不排除重复项，这样速度更快。
