SQL Server/T-SQL/Subquery/Subqueries Exists
Содержание
EXISTS evaluates to true if its result contains at least one row
<source lang="sql">
3> 4> CREATE TABLE works_on (emp_no INTEGER NOT NULL, 5> project_no CHAR(4) NOT NULL, 6> job CHAR (15) NULL, 7> enter_date DATETIME NULL) 8> 9> insert into works_on values (1, "p1", "analyst", "1997.10.1") 10> insert into works_on values (1, "p3", "manager", "1999.1.1") 11> insert into works_on values (2, "p2", "clerk", "1998.2.15") 12> insert into works_on values (2, "p2", NULL, "1998.6.1") 13> insert into works_on values (3, "p2", NULL, "1997.12.15") 14> insert into works_on values (4, "p3", "analyst", "1998.10.15") 15> insert into works_on values (5, "p1", "manager", "1998.4.15") 16> insert into works_on values (6, "p1", NULL, "1998.8.1") 17> insert into works_on values (7, "p2", "clerk", "1999.2.1") 18> insert into works_on values (8, "p3", "clerk", "1997.11.15") 19> insert into works_on values (7, "p1", "clerk", "1998.1.4") 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) 1> 2> -- EXISTS evaluates to true if its result contains at least one row 3> 4> SELECT job FROM works_on WHERE exists (SELECT emp_no FROM employee) 5> GO job
analyst manager clerk NULL NULL analyst manager NULL clerk clerk clerk (11 rows affected) 1> 2> drop table works_on 3> GO 1>
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EXISTS Function and Subqueries
<source lang="sql">
17> 18> CREATE TABLE employee (emp_no INTEGER NOT NULL, 19> emp_fname CHAR(20) NOT NULL, 20> emp_lname CHAR(20) NOT NULL, 21> dept_no CHAR(4) NULL) 22> 23> insert into employee values(1, "Matthew", "Smith", "d3") 24> insert into employee values(2, "Ann", "Jones", "d3") 25> insert into employee values(3, "John", "Barrimore","d1") 26> insert into employee values(4, "James", "James", "d2") 27> insert into employee values(5, "Elsa", "Bertoni", "d2") 28> insert into employee values(6, "Elke", "Hansel", "d2") 29> insert into employee values(7, "Sybill", "Moser", "d1") 30> 31> select * from employee 32> 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> -- EXISTS Function and Subqueries 2> 3> -- EXISTS function takes a subquery as an argument and returns true if the subquery returns one or more rows, and it returns false if the subquery returns zero rows. 4> 5> SELECT emp_lname FROM employee WHERE EXISTS 6> (SELECT * FROM works_on WHERE employee.emp_no = works_on.emp_no AND project_no="p1") 7> 8> drop table works_on 9> drop table employee 10> GO emp_lname
Smith Bertoni Hansel Moser (4 rows affected) 1>
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NOT EXISTS(): Add the NOT operator before the EXISTS statement
<source lang="sql">
1> create table employee( 2> ID int, 3> name nvarchar (10), 4> salary int ) 5> GO 1> 2> create table job( 3> ID int, 4> title nvarchar (10), 5> averageSalary int) 6> GO 1> 2> 3> insert into employee (ID, name, salary) values (1, "Jason", 1234) 4> GO (1 rows affected) 1> insert into employee (ID, name, salary) values (2, "Robert", 4321) 2> GO (1 rows affected) 1> insert into employee (ID, name, salary) values (3, "Celia", 5432) 2> GO (1 rows affected) 1> insert into employee (ID, name, salary) values (4, "Linda", 3456) 2> GO (1 rows affected) 1> insert into employee (ID, name, salary) values (5, "David", 7654) 2> GO (1 rows affected) 1> insert into employee (ID, name, salary) values (6, "James", 4567) 2> GO (1 rows affected) 1> insert into employee (ID, name, salary) values (7, "Alison", 8744) 2> GO (1 rows affected) 1> insert into employee (ID, name, salary) values (8, "Chris", 9875) 2> GO (1 rows affected) 1> insert into employee (ID, name, salary) values (9, "Mary", 2345) 2> GO (1 rows affected) 1> 2> insert into job(ID, title, averageSalary) values(1,"Developer",3000) 3> GO (1 rows affected) 1> insert into job(ID, title, averageSalary) values(2,"Tester", 4000) 2> GO (1 rows affected) 1> insert into job(ID, title, averageSalary) values(3,"Designer", 5000) 2> GO (1 rows affected) 1> insert into job(ID, title, averageSalary) values(4,"Programmer", 6000) 2> GO (1 rows affected) 1> 2> 3> select * from employee; 4> GO ID name salary
---------- -----------
1 Jason 1234
2 Robert 4321
3 Celia 5432
4 Linda 3456
5 David 7654
6 James 4567
7 Alison 8744
8 Chris 9875
9 Mary 2345
(9 rows affected) 1> select * from job; 2> GO ID title averageSalary
---------- -------------
1 Developer 3000
2 Tester 4000
3 Designer 5000
4 Programmer 6000
(4 rows affected) 1> 2> 3> -- NOT EXISTS() Add the NOT operator before the EXISTS statement: 4> 5> SELECT ID, Name 6> FROM Employee 7> WHERE NOT EXISTS 8> (SELECT * FROM Job 9> WHERE ID = Employee.ID) 10> GO ID Name
----------
5 David
6 James
7 Alison
8 Chris
9 Mary
(5 rows affected) 1> 2> 3> 4> drop table employee; 5> drop table job; 6> GO 1>
</source>
Using the EXISTS() Function
<source lang="sql">
1> 2> create table employee( 3> ID int, 4> name nvarchar (10), 5> salary int ) 6> GO 1> 2> create table job( 3> ID int, 4> title nvarchar (10), 5> averageSalary int) 6> GO 1> 2> 3> insert into employee (ID, name, salary) values (1, "Jason", 1234) 4> GO (1 rows affected) 1> insert into employee (ID, name, salary) values (2, "Robert", 4321) 2> GO (1 rows affected) 1> insert into employee (ID, name, salary) values (3, "Celia", 5432) 2> GO (1 rows affected) 1> insert into employee (ID, name, salary) values (4, "Linda", 3456) 2> GO (1 rows affected) 1> insert into employee (ID, name, salary) values (5, "David", 7654) 2> GO (1 rows affected) 1> insert into employee (ID, name, salary) values (6, "James", 4567) 2> GO (1 rows affected) 1> insert into employee (ID, name, salary) values (7, "Alison", 8744) 2> GO (1 rows affected) 1> insert into employee (ID, name, salary) values (8, "Chris", 9875) 2> GO (1 rows affected) 1> insert into employee (ID, name, salary) values (9, "Mary", 2345) 2> GO (1 rows affected) 1> 2> insert into job(ID, title, averageSalary) values(1,"Developer",3000) 3> GO (1 rows affected) 1> insert into job(ID, title, averageSalary) values(2,"Tester", 4000) 2> GO (1 rows affected) 1> insert into job(ID, title, averageSalary) values(3,"Designer", 5000) 2> GO (1 rows affected) 1> insert into job(ID, title, averageSalary) values(4,"Programmer", 6000) 2> GO (1 rows affected) 1> 2> 3> select * from employee; 4> GO ID name salary
---------- -----------
1 Jason 1234
2 Robert 4321
3 Celia 5432
4 Linda 3456
5 David 7654
6 James 4567
7 Alison 8744
8 Chris 9875
9 Mary 2345
(9 rows affected) 1> select * from job; 2> GO ID title averageSalary
---------- -------------
1 Developer 3000
2 Tester 4000
3 Designer 5000
4 Programmer 6000
(4 rows affected) 1> 2> 3> -- Using the EXISTS() Function 4> 5> -- It doesn"t really matter what column or columns are returned in the subquery because you don"t actually use these values. 6> 7> SELECT ID, Name 8> FROM Employee 9> WHERE EXISTS 10> (SELECT * FROM Job 11> WHERE ID = Employee.ID) 12> GO ID Name
----------
1 Jason
2 Robert
3 Celia
4 Linda
(4 rows affected) 1> 2> drop table employee; 3> drop table job; 4> GO 1>
</source>
