What does this query do ?

Question

There's a part of query i don't understand it

Can some explain it please?

(@a:=concat(@a,schema_name,' ')

If we consider that schema_name return all the databases name

Is it loop or what i can't understand

It's a lousy bit of code IMO. This kind of data display issue could be handled in the css. — Strawberry, Jul 31 '16 at 07:23

Gordon Linoff · Accepted Answer · 2016-08-01T01:12:41.103

0

Presumably, that is a statement in a select:

select @a := concat(@a, schema_name, '<br>')
from t;

If @a is initialized to NULL, then this returns NULL. Typically, it would be initialized to an empty string ('').

If so, it constructs a string containing all the values of schema_name followed by  . So, if the table contained:

schema_name
     a
     b
     c

The result would be 'a b c ' in the @a variable (and MySQL will return this result as well).

In MySQL, you would normally use group_concat() for this purpose.

edited Aug 01 '16 at 01:12

answered Jul 31 '16 at 03:08

Gordon Linoff

Ok , but can you tell me what does `:=` do ? – dragon Jul 31 '16 at 14:58
`a
b
c
` will be `@a` value ? – dragon Jul 31 '16 at 15:14
@dragon . . . Yes. The `:=` assigns values to variables in a `select`, `update`, or `delete` statement. – Gordon Linoff Aug 01 '16 at 01:13
is there any loop in this query ? – dragon Aug 01 '16 at 16:06
http://stackoverflow.com/questions/38704555/mysql-what-does-this-query-mean – dragon Aug 01 '16 at 17:16

1 Answers1