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MySQL > select * from test1 join test2 where test1.a=test2.a;     
+---+------+---------+---+------+---------------+
| a | b    | c       | a | d    | e             |
+---+------+---------+---+------+---------------+
| 1 |    2 | record1 | 1 |    3 | test2_record1 |
| 2 |    3 | record2 | 2 |    3 | test2_record2 |
+---+------+---------+---+------+---------------+
2 rows in set (0.00 sec)

MySQL > select * from test1 left join test2 on test1.a<test2.a where test1.a=1;
+---+------+---------+------+------+---------------+
| a | b    | c       | a    | d    | e             |
+---+------+---------+------+------+---------------+
| 1 |    2 | record1 |    2 |    3 | test2_record2 |
+---+------+---------+------+------+---------------+
1 row in set (0.00 sec)

MySQL> select * from test1 where test1.a in (select a from test2);        
+---+------+---------+
| a | b    | c       |
+---+------+---------+
| 1 |    2 | record1 |
| 2 |    3 | record2 |
+---+------+---------+
2 rows in set (0.00 sec)

MySQL> select a, count(1) from test1 where exists (select * from test2 where test2.a=test1.a) group by a;        
+---+----------+
| a | count(1) |
+---+----------+
| 1 |        1 |
| 2 |        1 |
+---+----------+
2 rows in set (0.00 sec)

MySQL> select distinct count(1) from test1 where exists (select * from test2 where test2.a=test1.a) group by a;   
+----------+
| count(1) |
+----------+
|        1 |
+----------+
1 row in set (0.00 sec)

MySQL> select count(distinct a) from test1 where exists (select * from test2 where test2.a=test1.a);        
+-------------------+
| count(distinct a) |
+-------------------+
|                 2 |
+-------------------+
1 row in set (0.00 sec)

mysql> create table noshard_table ( a int, b int key);
Query OK, 0 rows affected (0.02 sec)

mysql> create table noshard_table_2 ( a int, b int key);
Query OK, 0 rows affected (0.00 sec)

mysql> select * from noshard_table,noshard_table_2;
Empty set (0.00 sec)

mysql> insert into noshard_table (a,b) values(1,2),(3,4);
Query OK, 2 rows affected (0.00 sec)
Records: 2  Duplicates: 0  Warnings: 0

mysql> insert into noshard_table_2 (a,b) values(10,20),(30,40);
Query OK, 2 rows affected (0.00 sec)
Records: 2  Duplicates: 0  Warnings: 0

mysql> select * from noshard_table,noshard_table_2;
+------+---+------+----+
| a    | b | a    | b  |
+------+---+------+----+
|    1 | 2 |   10 | 20 |
|    3 | 4 |   10 | 20 |
|    1 | 2 |   30 | 40 |
|    3 | 4 |   30 | 40 |
+------+---+------+----+
4 rows in set (0.00 sec)

 MySQL> create table global_test(a int key, b int)shardkey=noshardkey_allset;
Query OK, 0 rows affected (0.00 sec)

MySQL> insert into global_test(a, b) values(1,1),(2,2);
Query OK, 2 rows affected (0.00 sec)

MySQL> select * from test1, global_test;
+---+------+---------+---+------+
| a | b    | c       | a | b    |
+---+------+---------+---+------+
| 1 |    2 | record1 | 1 |    1 |
| 2 |    3 | record2 | 1 |    1 |
| 1 |    2 | record1 | 2 |    2 |
| 2 |    3 | record2 | 2 |    2 |
+---+------+---------+---+------+
4 rows in set (0.00 sec)

mysql> select a from (select * from test1 where a=1) as t;
+---+
| a |
+---+
| 1 |
+---+
1 row in set (0.00 sec)

mysql> create table test1 ( a int key, b int, c char(20) ) shardkey=a;
Query OK, 0 rows affected (1.56 sec)

mysql> create table test2 ( a int key, d int, e char(20) ) shardkey=a;
Query OK, 0 rows affected (1.46 sec)

mysql> insert into test1 (a,b,c) values(1,2,"record1"),(2,3,"record2");
Query OK, 2 rows affected (0.02 sec)

mysql> insert into test2 (a,d,e) values(1,3,"test2_record1"),(2,3,"test2_record2");
Query OK, 2 rows affected (0.02 sec)

mysql> select * from test1 join test2 on test1.b=test2.d;
+---+------+---------+---+------+---------------+
| a | b    | c       | a | d    | e             |
+---+------+---------+---+------+---------------+
| 2 |    3 | record2 | 1 |    3 | test2_record1 |
| 2 |    3 | record2 | 2 |    3 | test2_record2 |
+---+------+---------+---+------+---------------+
2 rows in set (0.00 sec)

MySQL> select * from test1 where exists (select * from test2 where test2.a=test1.b);
+---+------+---------+
| a | b    | c       |
+---+------+---------+
| 1 |    2 | record1 |
+---+------+---------+
1 row in set (0.00 sec)

MySQL [th]> update test1 set test1.c="record" where exists(select 1 from test2 where test1.b=test2.d);
Query OK, 1 row affected (0.00 sec)

MySQL [th]> update test1, test2 set test1.b=2 where test1.b=test2.d;
Query OK, 1 row affected (0.00 sec)

MySQL [th]> insert into test1 select cast(rand()*1024 as unsigned), d, e from test2;
Query OK, 2 rows affected (0.00 sec)

MySQL [th]> delete from test1 where b in (select b from test2);
Query OK, 6 rows affected (0.00 sec)

MySQL [th]> delete from test2.* using test1 right join test2 on test1.a=test2.a where test1.a is null;
Query OK, 2 rows affected (0.00 sec)