What does the := operator mean in mysql?

Question

I have a mysql table (scho_id,school_name,school_views).

I was looking for a mysql query to get rank of schools on the basis of school_views.

I found this solution on stackoverflow.

SET @points := -1, @num := 0;
SELECT scho_id
, school_views
, @num := if(@points = school_views, @num, @num + 1) as school_rank
, @points := school_info.school_views as dummy
FROM school_info
ORDER BY school_views desc, scho_id asc;

This solved my problem but I notice a new operator := in this query. I am curious to know the meaning and uses of this operator.

Solution

In MySQL, := is an assignment operator:

SELECT @foo := 'bar';    // variable 'foo' now has value 'bar'
return value: 'bar'

while = is an equality test:

SELECT @foo = 'hi mom'; // does variable 'foo' have the value 'hi mom';
return value: false   ('bar' == 'hi mom' -> false)

Note that you CAN do both equality testing AND assignment with set queries:

SET @foo = 'bar' = 'baz';

which will cause @foo to be assigned false, the boolean result of 'bar' = 'baz'. It executes as the following:

SET @foo = ('bar' = 'baz');
SET @foo = false;

Comments

[Joe Phillips]

If you simply check the mysql docs (dev.mysql.com/doc/refman/8.0/en/…) you will see that..... they're wrong. Nice.

[prograshid]

So the = (equality test operator ) work as assignment operator if it is used with SET ? E.g - SET @param='some value' and with UPDATE statement e.g. UPDATE table_name SET column_name='value'

[Marc B]

yes. the first = in a set does assignment. a set query can't return anything, so it doesn't make much sense to have = do an equality test. the result of that test couldn't be returned. however, now that this will do testing AND assignment: set @foo = 'bar' = 'baz';. @foo will be assignment the boolean result of the 'bar' = 'baz' equality test, e.g. false.

[Timofeus]

It seem though, you cannot use := with declared variables in SELECT statements.