Subquery can be any SELECT statement, directly related or not, to the main SELECT statement.
EXISTS() is often used in correlated subqueries to either mimic the behavior of a join.
It doesn't really matter what column or columns are returned in the subquery because you don't actually use these values.
9> CREATE TABLE employee(
10> id INTEGER NOT NULL PRIMARY KEY,
11> first_name VARCHAR(10),
12> last_name VARCHAR(10),
13> salary DECIMAL(10,2),
14> start_Date DATETIME,
15> region VARCHAR(10),
16> city VARCHAR(20)
17> );
18> GO
1> INSERT INTO employee VALUES (1, 'Jason' , 'Martin', 5890,'2005-03-22','North','Vancouver');
2> INSERT INTO employee VALUES (2, 'Alison', 'Mathews',4789,'2003-07-21','South','Utown');
3> INSERT INTO employee VALUES (3, 'James' , 'Smith', 6678,'2001-12-01','North','Paris');
4> INSERT INTO employee VALUES (4, 'Celia' , 'Rice', 5567,'2006-03-03','South','London');
5> INSERT INTO employee VALUES (5, 'Robert', 'Black', 4467,'2004-07-02','East','Newton');
6> INSERT INTO employee VALUES (6, 'Linda' , 'Green' , 6456,'2002-05-19','East','Calgary');
7> INSERT INTO employee VALUES (7, 'David' , 'Larry', 5345,'2008-03-18','West','New York');
8> INSERT INTO employee VALUES (8, 'James' , 'Cat', 4234,'2007-07-17','West','Regina');
9> INSERT INTO employee VALUES (9, 'Joan' , 'Act', 6123,'2001-04-16','North','Toronto');
10> 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> select * from employee;
2> GO
id first_name last_name salary start_Date region city
----------- ---------- ---------- ------------ ----------------------- ---------- --------------------
1 Jason Martin 5890.00 2005-03-22 00:00:00.000 North Vancouver
2 Alison Mathews 4789.00 2003-07-21 00:00:00.000 South Utown
3 James Smith 6678.00 2001-12-01 00:00:00.000 North Paris
4 Celia Rice 5567.00 2006-03-03 00:00:00.000 South London
5 Robert Black 4467.00 2004-07-02 00:00:00.000 East Newton
6 Linda Green 6456.00 2002-05-19 00:00:00.000 East Calgary
7 David Larry 5345.00 2008-03-18 00:00:00.000 West New York
8 James Cat 4234.00 2007-07-17 00:00:00.000 West Regina
9 Joan Act 6123.00 2001-04-16 00:00:00.000 North Toronto
(9 rows affected)
1>
2> CREATE TABLE title(
3> id INTEGER,
4> job_title VARCHAR(20)
5> );
6> GO
1> INSERT INTO title VALUES (1, 'developer');
2> INSERT INTO title VALUES (1, 'manager');
3> INSERT INTO title VALUES (2, 'tester');
4> INSERT INTO title VALUES (2, 'programmer');
5> INSERT INTO title VALUES (3, 'boss');
6> INSERT INTO title VALUES (4, 'sales');
7> INSERT INTO title VALUES (5, 'market');
8> INSERT INTO title VALUES (6, 'coder');
9> INSERT INTO title VALUES (7, 'tester');
10> INSERT INTO title VALUES (8, 'developer');
11> INSERT INTO title VALUES (9, 'manager');
12> 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)
1>
2>
3> SELECT ID, First_Name, Last_Name
4> FROM Employee
5> WHERE EXISTS
6> (SELECT * FROM Title
7> WHERE ID = 1)
8>
9> drop table employee;
10> drop table title;
11> GO
ID First_Name Last_Name
----------- ---------- ----------
1 Jason Martin
2 Alison Mathews
3 James Smith
4 Celia Rice
5 Robert Black
6 Linda Green
7 David Larry
8 James Cat
9 Joan Act
(9 rows affected)
1>