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32 lines
1.3 KiB
Text
32 lines
1.3 KiB
Text
set @a := foo;
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You may only use constant expressions with SET
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set @a := connection_id() + 3;
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select @a - connection_id();
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@a - connection_id()
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3
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drop table if exists t1,t2;
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CREATE TABLE t1 ( i int not null, v int not null,index (i));
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insert into t1 values (1,1),(1,3),(2,1);
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create table t2 (i int not null, unique (i));
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insert into t2 select distinct i from t1;
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select * from t2;
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i
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1
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2
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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;
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i @vv1:=if(sv1.i,1,0) @vv2:=if(sv2.i,1,0) @vv3:=if(sv3.i,1,0) @vv1+@vv2+@vv3
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1 1 0 1 2
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2 1 0 0 1
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explain select * from t1 where i=@vv1;
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id select_type table type possible_keys key key_len ref rows Extra
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1 FIRST t1 ref i i 4 const 1 where used
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explain select * from t1 where @vv1:=@vv1+1 and i=@vv1;
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id select_type table type possible_keys key key_len ref rows Extra
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1 FIRST t1 ALL NULL NULL NULL NULL 3 where used
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explain select @vv1:=i from t1 where i=@vv1;
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id select_type table type possible_keys key key_len ref rows Extra
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1 FIRST t1 index NULL i 4 NULL 3 where used; Using index
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explain select * from t1 where i=@vv1;
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id select_type table type possible_keys key key_len ref rows Extra
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1 FIRST t1 ref i i 4 const 1 where used
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drop table t1,t2;
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