sql - MySQL assign user-defined variable in where clause -

the 1st query sql converted to:

select name,(@num)  test 1 

in select clause, (@num : @num + 1) return 1, mean last query sql equal to:

select name,(@num)  test 1 <= 1 

?

if yes, why 2nd query return first record?

if no, (@num := @num + 1) in where clause?

why @num in 3rd query 4?

create table test (   id int(1),   name varchar(10) );  insert test (id, name) values (1, 'a'), (2, 'b'), (3, 'c'), (4, 'd');  set @num := 0; select name, @num test (@num := 1) <= 1; -- return records.  set @num := 0; select name, @num test (@num := @num + 1) <= 1; -- return first record.  set @num := 0; select name, @num test (@num := @num + 1) <= 1 order name; -- return 1 record, name = a, @num = 4 

case 1st query:

the first query resolves following equivalent query:

set @num := 0; select name, @num test (@num := 1) <= 1;                 v set @num := 0; select name, @num test 1 <= 1;                 v set @num := 0; select name, @num test true; 

so, records. in every iteration 1 assigned @num. so, @num doesn't change.

case 2nd query:

in case of second query in first iteration resolves above query.

1st iteration:

select name, @num test (@num := @num + 1) <= 1;                      v @num 1 select name, @num test 1 <= 1; 

2nd iteration:

@num 2  select name, @num test 2 <= 1; 

3rd iteration:

@num 3     select name, @num test 3 <= 1; ........ .... , on 

look here first iteration qualifies condition in clause. others dropped.