MySQL的WHERE子句中的CASE或COALESCE性能

Question

我想知道在WHERE子句和NULL值中处理多个条件时哪个是更好的性能或最佳实践.

WHERE
    u.id = COALESCE(user_id, u.id) AND
    su.custom_id = COALESCE(student_number, su.custom_id)

要么

WHERE
CASE 
    WHEN user_id IS NOT NULL AND LENGTH(user_id) > 0
    THEN
        u.id = user_id 
    ELSE
        su.custom_id = student_number
    END

Answer 1

我会避免这两种方法.你应该能够完成你正在尝试与合理使用做AND,OR,IS NULL,和括号.

例如,您可以重写:

WHERE
    u.id = COALESCE(user_id, u.id) AND
    su.custom_id = COALESCE(student_number, su.custom_id)

像这样:

WHERE
    (user_id IS NULL OR u.id = user_id) AND
    (su.custom_id = student_number) 

你可以改写这个:

WHERE
CASE 
    WHEN user_id IS NOT NULL AND LENGTH(user_id) > 0
    THEN
        u.id = user_id 
    ELSE
        su.custom_id = student_number
    END

这样:

WHERE
(user_id IS NOT NULL AND LENGTH(user_id) > 0 AND u.id = user_id)
OR
(su.custom_id = student_number)