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select (select 2);
(select 2)
2
drop table if exists t1,t2,t3,t4,t5,attend,clinic;
create table t1 (a int);
create table t2 (a int, b int);
create table t3 (a int);
create table t4 (a int, b int);
insert into t1 values (2);
insert into t2 values (1,7),(2,7);
insert into t4 values (4,8),(3,8),(5,9);
select (select a from t1 where t1.a=t2.a), a from t2;
(select a from t1 where t1.a=t2.a) a
NULL 1
2 2
select (select a from t1 where t1.a=t2.b), a from t2;
(select a from t1 where t1.a=t2.b) a
NULL 1
NULL 2
select (select a from t1), a from t2;
(select a from t1) a
2 1
2 2
select (select a from t3), a from t2;
(select a from t3) a
NULL 1
NULL 2
select * from t2 where t2.a=(select a from t1);
a b
2 7
insert into t3 values (6),(7),(3);
select * from t2 where t2.b=(select a from t3 order by 1 desc limit 1);
a b
1 7
2 7
select * from t2 where t2.b=(select a from t3 order by 1 desc limit 1)
union (select * from t4 order by a limit 2) limit 3;
a b
1 7
2 7
3 8
select * from t2 where t2.b=(select a from t3 order by 1 desc limit 1)
union (select * from t4 where t4.b=(select max(t2.a)*4 from t2) order by a);
a b
1 7
2 7
3 8
4 8
explain select * from t2 where t2.b=(select a from t3 order by 1 desc limit 1)
union (select * from t4 where t4.b=(select max(t2.a)*4 from t2) order by a);
id select_type table type possible_keys key key_len ref rows Extra
1 FIRST t2 ALL NULL NULL NULL NULL 2 where used
3 UNION t4 ALL NULL NULL NULL NULL 3 where used; Using filesort
4 SUBSELECT t2 ALL NULL NULL NULL NULL 2
2 SUBSELECT t3 ALL NULL NULL NULL NULL 3 Using filesort
select (select a from t3 where a<t2.a*4 order by 1 desc limit 1), a from t2;
(select a from t3 where a<t2.a*4 order by 1 desc limit 1) a
3 1
7 2
select (select t3.a from t3 where a<8 order by 1 desc limit 1), a from
(select * from t2 where a>1) as tt;
(select t3.a from t3 where a<8 order by 1 desc limit 1) a
7 2
explain select (select t3.a from t3 where a<8 order by 1 desc limit 1), a from
(select * from t2 where a>1) as tt;
id select_type table type possible_keys key key_len ref rows Extra
3 DERIVED t2 ALL NULL NULL NULL NULL 2 where used
1 FIRST <derived3> system NULL NULL NULL NULL 1
2 SUBSELECT t3 ALL NULL NULL NULL NULL 3 where used; Using filesort
select * from t1 where t1.a=(select t2.a from t2 where t2.b=(select max(a) from t3) order by 1 desc limit 1);
a
2
select * from t1 where t1.a=(select t2.a from t2 where t2.b=(select max(a) from t3 where t3.a > t1.a) order by 1 desc limit 1);
a
2
select * from t1 where t1.a=(select t2.a from t2 where t2.b=(select max(a) from t3 where t3.a < t1.a) order by 1 desc limit 1);
a
select b,(select avg(t2.a+(select min(t3.a) from t3 where t3.a >= t4.a)) from t2) from t4;
b (select avg(t2.a+(select min(t3.a) from t3 where t3.a >= t4.a)) from t2)
8 7.5000
8 4.5000
9 7.5000
explain select b,(select avg(t2.a+(select min(t3.a) from t3 where t3.a >= t4.a)) from t2) from t4;
id select_type table type possible_keys key key_len ref rows Extra
1 FIRST t4 ALL NULL NULL NULL NULL 3
2 DEPENDENT SUBSELECT t2 ALL NULL NULL NULL NULL 2
3 DEPENDENT SUBSELECT t3 ALL NULL NULL NULL NULL 3 where used
select * from t3 where exists (select * from t2 where t2.b=t3.a);
a
7
select * from t3 where not exists (select * from t2 where t2.b=t3.a);
a
6
3
insert into t4 values (12,7),(1,7),(10,9),(9,6),(7,6),(3,9);
select b,max(a) as ma from t4 group by b having b < (select max(t2.a)
from t2 where t2.b=t4.b);
b ma
select b,max(a) as ma from t4 group by b having b >= (select max(t2.a)
from t2 where t2.b=t4.b);
b ma
7 12
create table t5 (a int);
select (select a from t1 where t1.a=t2.a union select a from t5 where t5.a=t2.a), a from t2;
(select a from t1 where t1.a=t2.a union select a from t5 where t5.a=t2.a) a
NULL 1
2 2
insert into t5 values (5);
select (select a from t1 where t1.a=t2.a union select a from t5 where t5.a=t2.a), a from t2;
(select a from t1 where t1.a=t2.a union select a from t5 where t5.a=t2.a) a
NULL 1
2 2
insert into t5 values (2);
select (select a from t1 where t1.a=t2.a union select a from t5 where t5.a=t2.a), a from t2;
(select a from t1 where t1.a=t2.a union select a from t5 where t5.a=t2.a) a
NULL 1
2 2
explain select (select a from t1 where t1.a=t2.a union select a from t5 where t5.a=t2.a), a from t2;
id select_type table type possible_keys key key_len ref rows Extra
1 FIRST t2 ALL NULL NULL NULL NULL 2
2 DEPENDENT SUBSELECT t1 system NULL NULL NULL NULL 1 where used
3 DEPENDENT UNION t5 ALL NULL NULL NULL NULL 2 where used
select (select a from t1 where t1.a=t2.a union all select a from t5 where t5.a=t2.a), a from t2;
Subselect return more than 1 record
create table attend (patient_uq int, clinic_uq int, index i1 (clinic_uq));
create table clinic( uq int primary key, name char(25));
insert into clinic values(1,"Oblastnaia bolnitsa"),(2,"Bolnitsa Krasnogo Kresta");
insert into attend values (1,1),(1,2),(2,2),(1,3);
select * from attend where exists (select * from clinic where uq = clinic_uq);
patient_uq clinic_uq
1 1
1 2
2 2
drop table t1,t2,t3,t4,t5,attend,clinic;
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