计算 SQL 服务器列中的 NULL 数

Question

I have two queries like so:

SELECT MyId, MyColumn FROM MyTable WHERE MyColumn IS NULL;
SELECT count(MyColumn) as MyCount FROM MyTable WHERE MyColumn IS NULL;

The results I get are:

MyId    MyColumn
10      NULL

Why is the count 0 always in the second query?

Answer 1

The COUNT() function ignores NULL values, and so the count in your second query will always be zero. Either count something else:

SELECT COUNT(*) AS MyCount
FROM MyTable
WHERE MyColumn IS NULL;

Or else count over the entire table using a CASE expression to explicitly count NULL values:

SELECT COUNT(CASE WHEN MyColumn IS NULL THEN 1 END) AS MyCount
FROM MyTable;

Answer 2

Count doesn't count null. You need to do something like this, transform null to 1 then sum them:

SELECT SUM(CASE WHEN MyColumn IS NULL THEN 1 ELSE 0 END) AS count_nulls
FROM MyTable;

Answer 3

You can simply use count(1) rather than column name in the count function as it ignores null value which counting.

SELECT COUNT(1) AS MyCount
FROM MyTable
WHERE MyColumn IS NULL;

Answer 4

As noted, COUNT(SomeValue) just counts the number of non-nulls, so you actually needed COUNT(*) .

But another way is to subtract the non-nulls from the total

SELECT COUNT(*) - COUNT(MyColumn) AS MyCount
FROM MyTable;

A WHERE is probably faster though, especially if you have an index on that column.