SQL Server/T-SQL/Subquery/Correlated subquery

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

   <source lang="sql">

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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Correlated subquery using Distinct

   <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> -- Correlated subquery using Distinct: 4> SELECT ID 5> , Name 6> FROM Employee AS e 7> WHERE 1 = 8> (SELECT DISTINCT ID 9> FROM Job As j 10> WHERE j.ID = e.ID) 11> 12> 13> 14> drop table employee; 15> drop table job; 16> GO ID Name


----------
         1 Jason

(1 rows affected) 1> 2>

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Correlated subquery using the department table in both inner and outer queries

   <source lang="sql">

1> 2> CREATE TABLE department(dept_no CHAR(4) NOT NULL, 3> dept_name CHAR(25) NOT NULL, 4> location CHAR(30) NULL) 5> 6> insert into department values ("d1", "developer", "Dallas") 7> insert into department values ("d2", "tester", "Seattle") 8> insert into department values ("d3", "marketing", "Dallas") 9> 10> select * from department 11> GO (1 rows affected) (1 rows affected) (1 rows affected) dept_no dept_name location


------------------------- ------------------------------

d1 developer Dallas d2 tester Seattle d3 marketing Dallas (3 rows affected) 1> -- Correlated subquery using the department table in both inner and outer queries 2> 3> SELECT t1.* 4> FROM department t1 5> WHERE t1.location IN 6> (SELECT t2.location 7> FROM department t2 8> WHERE t1.dept_no <> t2.dept_no) 9> GO dept_no dept_name location


------------------------- ------------------------------

d1 developer Dallas d3 marketing Dallas (2 rows affected) 1> 2> drop table department 3> GO 1>

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Leaf-Level Employees (Employees with No Subordinates), Correlated Subquery Syntax

   <source lang="sql">

5> 6> 7> 8> CREATE TABLE Employees 9> ( 10> empid int NOT NULL, 11> mgrid int NULL, 12> empname varchar(25) NOT NULL, 13> salary money NOT NULL) 14> GO 1> INSERT INTO employees(empid, mgrid, empname, salary) VALUES(1, NULL, "Nancy", $10000.00) 2> INSERT INTO employees(empid, mgrid, empname, salary) VALUES(2, 1, "Andrew", $5000.00) 3> INSERT INTO employees(empid, mgrid, empname, salary) VALUES(3, 1, "Janet", $5000.00) 4> INSERT INTO employees(empid, mgrid, empname, salary) VALUES(4, 1, "Margaret", $5000.00) 5> INSERT INTO employees(empid, mgrid, empname, salary) VALUES(5, 2, "Steven", $2500.00) 6> INSERT INTO employees(empid, mgrid, empname, salary) VALUES(6, 2, "Michael", $2500.00) 7> INSERT INTO employees(empid, mgrid, empname, salary) VALUES(7, 3, "Robert", $2500.00) 8> INSERT INTO employees(empid, mgrid, empname, salary) VALUES(8, 3, "Laura", $2500.00) 9> INSERT INTO employees(empid, mgrid, empname, salary) VALUES(9, 3, "Ann", $2500.00) 10> INSERT INTO employees(empid, mgrid, empname, salary) VALUES(10, 4, "Ina", $2500.00) 11> INSERT INTO employees(empid, mgrid, empname, salary) VALUES(11, 7, "David", $2000.00) 12> INSERT INTO employees(empid, mgrid, empname, salary) VALUES(12, 7, "Ron", $2000.00) 13> INSERT INTO employees(empid, mgrid, empname, salary) VALUES(13, 7, "Dan", $2000.00) 14> INSERT INTO employees(empid, mgrid, empname, salary) VALUES(14, 11, "James", $1500.00) 15> 16> 17> SELECT * 18> FROM 19> Employees AS M 20> WHERE 21> NOT EXISTS 22> ( 23> SELECT 24> empid 25> FROM 26> Employees AS E 27> WHERE 28> E.mgrid = M.empid 29> ) 30> 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 rows affected) (1 rows affected) (1 rows affected) empid mgrid empname salary


----------- ------------------------- ---------------------
         5           2 Steven                                2500.0000
         6           2 Michael                               2500.0000
         8           3 Laura                                 2500.0000
         9           3 Ann                                   2500.0000
        10           4 Ina                                   2500.0000
        12           7 Ron                                   2000.0000
        13           7 Dan                                   2000.0000
        14          11 James                                 1500.0000

(8 rows affected) 1> 2> 3> drop table Employees; 4> GO

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Leaf-Level Employees (Employees with No Subordinates), Join Syntax

   <source lang="sql">

3> 4> 5> 6> CREATE TABLE Employees 7> ( 8> empid int NOT NULL, 9> mgrid int NULL, 10> empname varchar(25) NOT NULL, 11> salary money NOT NULL) 12> GO 1> INSERT INTO employees(empid, mgrid, empname, salary) VALUES(1, NULL, "Nancy", $10000.00) 2> INSERT INTO employees(empid, mgrid, empname, salary) VALUES(2, 1, "Andrew", $5000.00) 3> INSERT INTO employees(empid, mgrid, empname, salary) VALUES(3, 1, "Janet", $5000.00) 4> INSERT INTO employees(empid, mgrid, empname, salary) VALUES(4, 1, "Margaret", $5000.00) 5> INSERT INTO employees(empid, mgrid, empname, salary) VALUES(5, 2, "Steven", $2500.00) 6> INSERT INTO employees(empid, mgrid, empname, salary) VALUES(6, 2, "Michael", $2500.00) 7> INSERT INTO employees(empid, mgrid, empname, salary) VALUES(7, 3, "Robert", $2500.00) 8> INSERT INTO employees(empid, mgrid, empname, salary) VALUES(8, 3, "Laura", $2500.00) 9> INSERT INTO employees(empid, mgrid, empname, salary) VALUES(9, 3, "Ann", $2500.00) 10> INSERT INTO employees(empid, mgrid, empname, salary) VALUES(10, 4, "Ina", $2500.00) 11> INSERT INTO employees(empid, mgrid, empname, salary) VALUES(11, 7, "David", $2000.00) 12> INSERT INTO employees(empid, mgrid, empname, salary) VALUES(12, 7, "Ron", $2000.00) 13> INSERT INTO employees(empid, mgrid, empname, salary) VALUES(13, 7, "Dan", $2000.00) 14> INSERT INTO employees(empid, mgrid, empname, salary) VALUES(14, 11, "James", $1500.00) 15> 16> 17> SELECT 18> M.* 19> FROM 20> Employees AS M 21> LEFT OUTER JOIN 22> Employees AS E ON M.empid = E.mgrid 23> WHERE 24> E.mgrid IS NULL 25> 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 rows affected) (1 rows affected) (1 rows affected) empid mgrid empname salary


----------- ------------------------- ---------------------
         5           2 Steven                                2500.0000
         6           2 Michael                               2500.0000
         8           3 Laura                                 2500.0000
         9           3 Ann                                   2500.0000
        10           4 Ina                                   2500.0000
        12           7 Ron                                   2000.0000
        13           7 Dan                                   2000.0000
        14          11 James                                 1500.0000

(8 rows affected) 1> 2> 3> drop table Employees; 4> GO 1>

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WHERE clause in the subquery"s SELECT statement links the inner query to the outer query

   <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> -- The WHERE clause in the subquery"s SELECT statement links the inner query to the outer query. 4> -- This makes the inner query a correlated subquery 5> 6> SELECT TOP 29 e.ID, e.name, (SELECT j.title FROM job j WHERE j.ID = e.ID) "title" 7> FROM Employee e 8> GO ID name title


---------- ----------
         1 Jason      Developer
         2 Robert     Tester
         3 Celia      Designer
         4 Linda      Programmer
         5 David      NULL
         6 James      NULL
         7 Alison     NULL
         8 Chris      NULL
         9 Mary       NULL

(9 rows affected) 1> 2> drop table employee; 3> drop table job; 4> GO 1>

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