drop table if exists t1,t2; set @a := foo; ERROR 42S22: Unknown column 'foo' in 'field list' set @a := connection_id() + 3; select @a - connection_id(); @a - connection_id() 3 set @b := 1; select @b; @b 1 CREATE TABLE t1 ( i int not null, v int not null,index (i)); insert into t1 values (1,1),(1,3),(2,1); create table t2 (i int not null, unique (i)); insert into t2 select distinct i from t1; select * from t2; i 1 2 select distinct t2.i,@vv1:=if(sv1.i,1,0),@vv2:=if(sv2.i,1,0),@vv3:=if(sv3.i,1,0), @vv1+@vv2+@vv3 from t2 left join t1 as sv1 on sv1.i=t2.i and sv1.v=1 left join t1 as sv2 on sv2.i=t2.i and sv2.v=2 left join t1 as sv3 on sv3.i=t2.i and sv3.v=3; i @vv1:=if(sv1.i,1,0) @vv2:=if(sv2.i,1,0) @vv3:=if(sv3.i,1,0) @vv1+@vv2+@vv3 1 1 0 1 2 2 1 0 0 1 explain select * from t1 where i=@vv1; id select_type table type possible_keys key key_len ref rows Extra 1 SIMPLE t1 ref i i 4 const 1 Using where explain select * from t1 where @vv1:=@vv1+1 and i=@vv1; id select_type table type possible_keys key key_len ref rows Extra 1 SIMPLE t1 ALL NULL NULL NULL NULL 3 Using where explain select @vv1:=i from t1 where i=@vv1; id select_type table type possible_keys key key_len ref rows Extra 1 SIMPLE t1 index NULL i 4 NULL 3 Using where; Using index explain select * from t1 where i=@vv1; id select_type table type possible_keys key key_len ref rows Extra 1 SIMPLE t1 ref i i 4 const 1 Using where drop table t1,t2; set @a=0,@b=0; select @a:=10, @b:=1, @a > @b, @a < @b; @a:=10 @b:=1 @a > @b @a < @b 10 1 1 0 select @a:="10", @b:="1", @a > @b, @a < @b; @a:="10" @b:="1" @a > @b @a < @b 10 1 1 0 select @a:=10, @b:=2, @a > @b, @a < @b; @a:=10 @b:=2 @a > @b @a < @b 10 2 0 1 select @a:="10", @b:="2", @a > @b, @a < @b; @a:="10" @b:="2" @a > @b @a < @b 10 2 1 0 select @a:=1; @a:=1 1 select @a, @a:=1; @a @a:=1 1 1 create table t1 (id int, d double, c char(10)); insert into t1 values (1,2.0, "test"); select @c:=0; @c:=0 0 update t1 SET id=(@c:=@c+1); select @c; @c 1 select @c:=0; @c:=0 0 update t1 set id=(@c:=@c+1); select @c; @c 1 select @c:=0; @c:=0 0 select @c:=@c+1; @c:=@c+1 1 select @d,(@d:=id),@d from t1; @d (@d:=id) @d NULL 1 1 select @e,(@e:=d),@e from t1; @e (@e:=d) @e NULL 2 2 select @f,(@f:=c),@f from t1; @f (@f:=c) @f NULL test test set @g=1; select @g,(@g:=c),@g from t1; @g (@g:=c) @g 1 test test select @c, @d, @e, @f; @c @d @e @f 1 1 2 test select @d:=id, @e:=id, @f:=id, @g:=@id from t1; @d:=id @e:=id @f:=id @g:=@id 1 1 1 NULL select @c, @d, @e, @f, @g; @c @d @e @f @g 1 1 1 1 NULL drop table t1; select @a:=10, @b:=2, @a>@b, @a:="10", @b:="2", @a>@b, @a:=10, @b:=2, @a>@b, @a:="10", @b:="2", @a>@b; @a:=10 @b:=2 @a>@b @a:="10" @b:="2" @a>@b @a:=10 @b:=2 @a>@b @a:="10" @b:="2" @a>@b 10 2 1 10 2 1 10 2 1 10 2 1