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[SQL] SELECT COUNT(*)
http://blog.jooq.org/2014/08/08/sql-tip-of-the-day-be-wary-of-select-count/
如果想睇下某條Record(s)存唔存在的話，條SQL可能會咁寫:
但係其實可以利用EXISTS呢個Keyword:
As you can see, the EXISTS predicate could be aborted early, as soon as the first record that matches the predicate is encountered – in this case immediately.
如果想睇下某條Record(s)存唔存在的話，條SQL可能會咁寫:
SELECT CASE WHEN COUNT(*) > 0 THEN TRUE
ELSE FALSE
END
FROM table_name
WHERE id = 123;
但係其實可以利用EXISTS呢個Keyword:
SELECT CASE WHEN EXISTS (
SELECT 1
FROM table_name
WHERE id = 123
) THEN TRUE
ELSE FALSE
END;
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