检查 postgresql 中是否存在一行
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Check if a row exists or not in postgresql
提问by Ashwin
I have seen many posts about this in SO. But I could not get an answer. I want to the query to check if a particular row exists or not in a table. If it exists, it should return me a stringtrue and stop the search there itself and if not return false.
我在 SO 中看到了很多关于这个的帖子。但我无法得到答案。我想查询以检查表中是否存在特定行。如果它存在，它应该返回一个字符串true 并在那里停止搜索，如果不存在则返回 false。
回答by Tometzky
select
case when exists (select true from table_name where table_column=?)
then 'true'
else 'false'
end;
But it would be better to just return boolean instead of string:
但最好只返回布尔值而不是字符串：
select exists (select true from table_name where table_column=?);
回答by wildplasser
Spoiler:
剧透：
-- EXPLAIN ANALYZE
WITH magic AS (
WITH lousy AS ( SELECT * FROM one WHERE num = -1)
SELECT 'True'::text AS truth
WHERE EXISTS (SELECT * FROM lousy)
UNION ALL
SELECT 'False'::text AS truth
WHERE NOT EXISTS (SELECT * FROM lousy)
)
SELECT *
FROM magic
;
