SQL Server/T-SQL/Table/Sub query

Subqueries: calculate an aggregate value on the fly and use it in the outer query for comparison

1>
2> CREATE TABLE works_on        (emp_no       INTEGER NOT NULL,
3>                         project_no    CHAR(4) NOT NULL,
4>                         job CHAR (15) NULL,
5>                         enter_date    DATETIME NULL)
6>
7> insert into works_on values (1, "p1", "analyst", "1997.10.1")
8> insert into works_on values (1, "p3", "manager", "1999.1.1")
9> insert into works_on values (2, "p2", "clerk",   "1998.2.15")
10> insert into works_on values (2, "p2",  NULL,     "1998.6.1")
11> insert into works_on values (3, "p2",  NULL,     "1997.12.15")
12> insert into works_on values (4, "p3", "analyst", "1998.10.15")
13> insert into works_on values (5, "p1", "manager", "1998.4.15")
14> insert into works_on values (6, "p1",  NULL,     "1998.8.1")
15> insert into works_on values (7, "p2", "clerk",   "1999.2.1")
16> insert into works_on values (8, "p3", "clerk",   "1997.11.15")
17> insert into works_on values (7, "p1", "clerk",   "1998.1.4")
18>
19> select * from works_on
20> GO
(1 rows affected)
(1 rows affected)
(1 rows affected)
(1 rows affected)
(1 rows affected)
(1 rows affected)
(1 rows affected)
(1 rows affected)
(1 rows affected)
(1 rows affected)
(1 rows affected)
emp_no      project_no job             enter_date
----------- ---------- --------------- -----------------------
          1 p1         analyst         1997-10-01 00:00:00.000
          1 p3         manager         1999-01-01 00:00:00.000
          2 p2         clerk           1998-02-15 00:00:00.000
          2 p2         NULL            1998-06-01 00:00:00.000
          3 p2         NULL            1997-12-15 00:00:00.000
          4 p3         analyst         1998-10-15 00:00:00.000
          5 p1         manager         1998-04-15 00:00:00.000
          6 p1         NULL            1998-08-01 00:00:00.000
          7 p2         clerk           1999-02-01 00:00:00.000
          8 p3         clerk           1997-11-15 00:00:00.000
          7 p1         clerk           1998-01-04 00:00:00.000
(11 rows affected)
1> -- Subquery Advantages
2>
3> -- Subqueries: calculate an aggregate value on the fly and use it in the outer query for comparison.
4>
5> SELECT emp_no, enter_date FROM works_on
6>         WHERE enter_date = (SELECT min(enter_date) FROM works_on)
7> GO
emp_no      enter_date
----------- -----------------------
          1 1997-10-01 00:00:00.000
(1 rows affected)
1>
2> drop table works_on
3> GO
1>
2>

Use aggregate function in sub-query

1> create table employee(
2>     ID          int,
3>     name        nvarchar (10),
4>     salary      int,
5>     start_date  datetime,
6>     city        nvarchar (10),
7>     region      char (1))
8> GO
1>
2> insert into employee (ID, name,    salary, start_date, city,       region)
3>               values (1,  "Jason", 40420,  "02/01/94", "New York", "W")
4> GO
(1 rows affected)
1> insert into employee (ID, name,    salary, start_date, city,       region)
2>               values (2,  "Robert",14420,  "01/02/95", "Vancouver","N")
3> GO
(1 rows affected)
1> insert into employee (ID, name,    salary, start_date, city,       region)
2>               values (3,  "Celia", 24020,  "12/03/96", "Toronto",  "W")
3> GO
(1 rows affected)
1> insert into employee (ID, name,    salary, start_date, city,       region)
2>               values (4,  "Linda", 40620,  "11/04/97", "New York", "N")
3> GO
(1 rows affected)
1> insert into employee (ID, name,    salary, start_date, city,       region)
2>               values (5,  "David", 80026,  "10/05/98", "Vancouver","W")
3> GO
(1 rows affected)
1> insert into employee (ID, name,    salary, start_date, city,       region)
2>               values (6,  "James", 70060,  "09/06/99", "Toronto",  "N")
3> GO
(1 rows affected)
1> insert into employee (ID, name,    salary, start_date, city,       region)
2>               values (7,  "Alison",90620,  "08/07/00", "New York", "W")
3> GO
(1 rows affected)
1> insert into employee (ID, name,    salary, start_date, city,       region)
2>               values (8,  "Chris", 26020,  "07/08/01", "Vancouver","N")
3> GO
(1 rows affected)
1> insert into employee (ID, name,    salary, start_date, city,       region)
2>               values (9,  "Mary",  60020,  "06/09/02", "Toronto",  "W")
3> GO
(1 rows affected)
1>
2> select * from employee
3> GO
ID          name       salary      start_date              city       region
----------- ---------- ----------- ----------------------- ---------- ------
          1 Jason            40420 1994-02-01 00:00:00.000 New York   W
          2 Robert           14420 1995-01-02 00:00:00.000 Vancouver  N
          3 Celia            24020 1996-12-03 00:00:00.000 Toronto    W
          4 Linda            40620 1997-11-04 00:00:00.000 New York   N
          5 David            80026 1998-10-05 00:00:00.000 Vancouver  W
          6 James            70060 1999-09-06 00:00:00.000 Toronto    N
          7 Alison           90620 2000-08-07 00:00:00.000 New York   W
          8 Chris            26020 2001-07-08 00:00:00.000 Vancouver  N
          9 Mary             60020 2002-06-09 00:00:00.000 Toronto    W
(9 rows affected)
1>
2> SELECT ID, name
3> FROM Employee
4> WHERE salary >
5>     (SELECT AVG (salary)
6>      FROM employee)
7> GO
ID          name
----------- ----------
          5 David
          6 James
          7 Alison
          9 Mary
(4 rows affected)
1>
2>
3> drop table employee
4> GO
1>