[英]What does the := operator mean in mysql?
I have a mysql
table ( scho_id
, school_name
, school_views
). 我有一个mysql
表( scho_id
, school_name
, school_views
)。
I was looking for a mysql
query to get rank
of schools on the basis of school_views
. 我正在寻找一个mysql
查询来根据school_views
获得学校rank
。
I found this solution on stackoverflow. 我在stackoverflow上找到了这个解决方案。
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. 这解决了我的问题,但我注意到一个新的运算符:=
在此查询中。 I am curious to know the meaning and uses of this operator. 我很想知道这个运算符的含义和用法。
In MySQL, :=
is an assignment operator: 在MySQL中, :=
是赋值运算符:
SELECT @foo := 'bar'; // variable 'foo' now has value 'bar'
return value: 'bar'
while =
is an equality test: while =
是一个相等测试:
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: 请注意,您可以使用set
查询执行相等性测试和赋值:
SET @foo = 'bar' = 'baz';
which will cause @foo
to be assigned false
, the boolean result of 'bar' = 'baz'
. 这将导致@foo
被赋值为false
,布尔结果为'bar' = 'baz'
。 It executes as the following: 它执行如下:
SET @foo = ('bar' = 'baz');
SET @foo = false;
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