SQL Server where子句中的where列为null

Question

Let's say that we have a table named Data with Id and Weather columns. Other columns in that table are not important to this problem. The Weather column can be null.

I want to display all rows where Weather fits a condition, but if there is a null value in weather then display null value.

My SQL so far:

SELECT * 
FROM Data d
WHERE (d.Weather LIKE '%'+COALESCE(NULLIF('',''),'sunny')+'%' OR d.Weather IS NULL)

My results are wrong, because that statement also shows values where Weather is null if condition is not correct (let's say that users mistyped wrong).

I found similar topic, but there I do not find appropriate answer. SQL WHERE clause not returning rows when field has NULL value

Please help me out.

Answer 1

Your query is correct for the general task of treating NULL s as a match. If you wish to suppress NULL s when there are no other results, you can add an AND EXISTS ... condition to your query, like this:

SELECT * 
FROM Data d
WHERE d.Weather LIKE '%'+COALESCE(NULLIF('',''),'sunny')+'%'
   OR (d.Weather IS NULL AND EXISTS (SELECT * FROM Data dd WHERE dd.Weather LIKE '%'+COALESCE(NULLIF('',''),'sunny')+'%'))

The additional condition ensures that NULL s are treated as matches only if other matching records exist.

You can also use a common table expression to avoid duplicating the query, like this:

WITH cte (id, weather) AS
(
    SELECT * 
    FROM Data d
    WHERE d.Weather LIKE '%'+COALESCE(NULLIF('',''),'sunny')+'%'
)
SELECT * FROM cte
UNION ALL
SELECT * FROM Data WHERE weather is NULL AND EXISTS (SELECT * FROM cte)

statement show also values where Wether is null if condition is not correct (let say that users typed wrong sunny).

This suggests that the constant 'sunny' is coming from end-user's input. If that is the case, you need to parameterize your query to avoid SQL injection attacks .