SQL @x：= @x +1和@x：= 0是什么意思？

Question

I was doing rank score problem at leetcode and I am not sure below solution. I can understand every part except for @x := @x +1 and @x := 0 .

select scores.score, ranks.rank from scores left join (
    select score, @x := @x +1 as rank from (select distinct score from scores order by score desc) s, (select @x := 0) r
) 
as ranks on scores.score = ranks.score order by scores.score desc;

Anyone could help please?

Answer 1

declare a var @x initialization it as int 0

select @x := 0

---

When you doing select clause the @x will add 1 .

select @x := @x +1

Here is a sample

Schema (MySQL v5.6)

CREATE TABLE T(
   col1 varchar(51)
);

INSERT INTO T VALUES ('TEST');
INSERT INTO T VALUES ('TEST1');

---

Query #1

SELECT  *,@x:=@x +1
FROM T  CROSS JOIN (select @x := 0) v;

| col1  | @x := 0 | @x:=@x +1 |
| ----- | ------- | --------- |
| TEST  | 0       | 1         |
| TEST1 | 0       | 2         |

---

View on DB Fiddle

NOTE

select score, @x := @x +1 as rank 
from (select distinct score from scores order by score desc) s, (select @x := 0) r

• the , comma between two tables in the query means CROSS JOIN