postgresql 如何选择列值为空的行？
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How to select rows having column value as null?
提问by afzalex
Consider following database table:
考虑以下数据库表：
id | is_visible
________|_______________
1 | true
2 | false
3 | -- i.e. null
I want to select all the rows having is_visible = null. I tried the following queries:
我想选择所有具有 is_visible = null 的行。我尝试了以下查询：
SELECT * FROM tab WHERE is_visible = null; -- 0 results
SELECT * FROM tab WHERE is_visible <> true; -- only fields with false value
SELECT * FROM tab WHERE is_visible <> false; -- only fields with true
I tried it in postgresql and sql server
How to select rows having is_visible = null?
我在 postgresql 和 sql server 中尝试过
如何选择具有 is_visible = null 的行？
回答by chaudharyp
In both Postgres and SQL server,
在 Postgres 和 SQL 服务器中，
SELECT * FROM tab WHERE is_visible is null;
If you want to select the rows for which column values are not null, then use is not nulloperator:
如果要选择列值不为空的行，请使用is not null运算符：
SELECT * FROM tab WHERE is_visible is not null;
回答by Sundar Singh
you can't compare 2 null values in sql server as null is unknown or not available. https://technet.microsoft.com/en-us/library/aa196339(v=sql.80).aspx
您无法比较 sql server 中的 2 个 null 值，因为 null 未知或不可用。 https://technet.microsoft.com/en-us/library/aa196339(v=sql.80).aspx
Use IS NULL operator:
使用 IS NULL 运算符：
SELECT * FROM tab WHERE is_visible IS NULL; -- only field having null value
SELECT * FROM tab WHERE is_visible IS NOT NULL; -- fields having not null values
