MySql 如何 COUNT(column_name = 'foobar' or null) 工作？

Question

I want to understand how or null works in this example:

Select columnA, count(columnB = 'foobar' or null) as result 
from orders 
group by columnA

If I don't use or null then it just give the count(*) of columnB based on group by but with or null it gives correct count of values where columnB = 'foobar'

Just wondering how it works internally?

Answer 1

Assuming that columnB is not nullable, the expression:

columnB = 'foobar'

is a boolean expression that is evaluated as 1 for true or 0 for false .

So by using it with COUNT() here:

count(columnB = 'foobar')

it is equivalent to count(0) or count(1) which both return the same result:

the number of all of the rows of the table (just like count(*) )

because the argument of COUNT() is never NULL .

The expression:

columnB = 'foobar' or null

is also a boolean expression, but it may also be evaluated as null when columnB = 'foobar' is false ( false or null is null , while true or null is true ).

So by using it with COUNT() here:

count(columnB = 'foobar' or null)

it counts only the rows where columnB = 'foobar' is true , because for all the others columnB = 'foobar' or null is null .

Although your code works, I prefer to use conditional aggregation, like:

count(case when columnB = 'foobar' then 1 end)

or:

sum(columnB = 'foobar') -- works in MySql and SQLite