postgresql - 检查postgresql中是否存在一行

Question

我在 SO 中看到了很多关于此的帖子。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 并自行停止搜索，如果不返回 false。

Answer 1

select
  case when exists (select true from table_name where table_column=?)
    then 'true'
    else 'false'
  end;

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=?);

Answer 2

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
        ;