Correlated subquery: the inner query depends on the outer query for any of its values : Correlated subquery « Subquery « SQL Server / T-SQL






Correlated subquery: the inner query depends on the outer query for any of its values


1> CREATE TABLE employee  (emp_no    INTEGER NOT NULL,
2>                         emp_fname CHAR(20) NOT NULL,
3>                         emp_lname CHAR(20) NOT NULL,
4>                         dept_no   CHAR(4) NULL)
5>
6> insert into employee values(1,  'Matthew', 'Smith',    'd3')
7> insert into employee values(2,  'Ann',     'Jones',    'd3')
8> insert into employee values(3,  'John',    'Barrimore','d1')
9> insert into employee values(4,  'James',   'James',    'd2')
10> insert into employee values(5,  'Elsa',    'Bertoni',  'd2')
11> insert into employee values(6,  'Elke',    'Hansel',   'd2')
12> insert into employee values(7,  'Sybill',  'Moser',    'd1')
13>
14> select * from employee
15> GO

(1 rows affected)

(1 rows affected)

(1 rows affected)

(1 rows affected)

(1 rows affected)

(1 rows affected)

(1 rows affected)
emp_no      emp_fname            emp_lname            dept_no
----------- -------------------- -------------------- -------
          1 Matthew              Smith                d3
          2 Ann                  Jones                d3
          3 John                 Barrimore            d1
          4 James                James                d2
          5 Elsa                 Bertoni              d2
          6 Elke                 Hansel               d2
          7 Sybill               Moser                d1

(7 rows affected)
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>
2>
3> -- Correlated Subqueries
4>
5> -- A subquery is said to be a correlated subquery if the inner query depends on the outer query for any of its values.
6>
7> SELECT emp_lname
8>        FROM employee
9>        WHERE emp_no IN
10>        (SELECT emp_no
11>          FROM works_on
12>          WHERE project_no='p3')
13> GO
emp_lname
--------------------
Smith
James

(2 rows affected)
1>
2> drop table works_on
3> drop table employee
4> GO
1>
           
       








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1.WHERE clause in the subquery's SELECT statement links the inner query to the outer query
2.Correlated subquery using the department table in both inner and outer queries
3.Correlated subquery using Distinct
4.Leaf-Level Employees (Employees with No Subordinates), Correlated Subquery Syntax
5.Leaf-Level Employees (Employees with No Subordinates), Join Syntax