SQL与可空值的等式/不等式比较 [英] SQL equality/inequality comparison with nullable values

问题描述

首先，kludge解决方案，sentinel方法（你的程序不能允许输入哨兵值是必须的）：

 选择coalesce（a，-2147483648）= coalesce（b，-2147483648）as is_equal  -  a postgresism 
  

让我们说你忘记阻塞你的程序的哨兵值，用户在B字段上输入-2147483648，A为null。上面的代码报告true，应该报告false，不应该报告true或null。

什么是最简洁的方法来比较可空字段上的相等？

解决方案

A == B只应报告true或false，而不管字段是否为空。可能 IS [NOT] DISTINCT FROM 会在这里帮助。 / p>

first take, kludge solution, sentinel approach(it's imperative that your program should not allow inputting of sentinel value):

 select coalesce(a, -2147483648) = coalesce(b, -2147483648) as is_equal -- a little postgresism

let's say you forgot to block the sentinel value on your program, the user inputted -2147483648 on the B field, and A is null. the code above reports true, should report false, should not report true nor null.

what's the most concise way to compare equality on nullable fields? A == B should just report either true or false only, regardless if the field(s) are nullable or not.

解决方案

Probably IS [NOT] DISTINCT FROM will help here.

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