SQL Joining

8/6/2019 SQL Joining

1/13

SQL JOINING


2/13

The JOIN keyword is used in an SQLstatement to query data from two or moretables, based on a relationship between

certain columns in these tables.

In joining A primary key is most importantarea of concern which is a column with a

unique value for each row. Each primary keyvalue must be unique within the table. Thepurpose is to bind data together, acrosstables, without repeating all of the data in

every table.


3/13

SQL JOIN

P_Id LastName FirstName Address City

P1 ADITY GANGWAR C.G. RAIPUR

P2 BASANT KUMAR C.G. RAIPUR

P3 CHAITANYA GUPTA C.G. BHILAI

P4 DEEPIKA KESHARWANI M.P. BHOPAL

P5 GURNIT KAUR DELHI RAJENDRA NAGAR

P6 JYOTI BAMNELE A.P. HYDRABAD

P7 LAO MANNA MAHARASHTRA MUMBAI

P8 MAHENDRA KALIHARI RAJASTHAN JAIPUR

The "P_Id" column is the primary key in the"Persons" table. This means that no two rowscan have the same P_Id. The P_Id distinguishestwo persons even if they have the same name.


4/13

SQL JOIN

O_Id OrderNo P_Id1 77895 P1

2 44678 P3

3 22456 P4

4 24562 P1

5 34764 P6

6 21543 P8

The "O_Id" column is the primary key in the"Orders" table and the "P_Id" column refers to thepersons in the "Persons" table without using theirnames. The relationship between the two tables

above is the "P_Id" column.


5/13

SQL JOIN Different types of joins

INNER JOIN: Return rows when there isat least one match in both tables.

LEFT JOIN: Return all rows from the left

table, even if there are no matches inthe right table.

RIGHT JOIN: Return all rows from the

right table, even if there are nomatches in the left table.

FULL JOIN: Return rows when there is a

match in one of the tables.


6/13

SQL JOIN

SQL INNER JOIN Syntax:

SELECT column_name(s) FROM table_name1 INNER JOIN table_name2 ONtable_name1.column_name = table_name2.column_name;

Example:SELECT Persons.LastName, Persons.FirstName, Orders.OrderNo FROM

Persons INNER JOIN Orders ON Persons.P_Id = Orders.P_Id;

SQL LEFT JOIN Syntax:SELECT column_name(s) FROM table_name1 LEFT JOIN table_name2 ONtable_name1.column_name = table_name2.column_name;

Example:SELECT Persons.LastName, Persons.FirstName, Orders.OrderNo FROMPersonsLEFT JOIN Orders ON Persons.P_Id = Orders.P_Id ORDER BYPersons.LastName


7/13

SQL JOIN

SQL RIGHT JOIN Syntax:SELECT column_name(s) FROM table_name1 RIGHT JOINtable_name2 ONtable_name1.column_name=table_name2.column_nameSELECT Persons.LastName, Persons.FirstName,

Orders.OrderNo FROM Persons RIGHT JOIN Orders ONPersons.P_Id=Orders.P_Id ORDER BY Persons.LastName;SQL FULL JOIN SyntaxSELECT column_name(s) FROM table_name1 FULL JOIN

table_name2 ONtable_name1.column_name=table_name2.column_name;Example:SELECT Persons.LastName, Persons.FirstName,Orders.OrderNo FROM Persons FULL JOIN Orders ON

Persons.P_Id=Orders.P_Id ORDER BY Persons.LastName;


8/13

SQL UNION

The SQL UNION Operator: The UNION operator is used tocombine the result-set of two or more SELECT statements.Each SELECT statement within the UNION must have thesame number of columns. The columns must also havesimilar data types. Also, the columns in each SELECT

statement must be in the same order.

SQL UNION SyntaxSELECT column_name(s) FROM table_name1 UNION SELECT column_name(s)FROM table_name2

Note: The UNION operator selects only distinct values by default. To allowduplicate values, use UNION ALL.

SQL UNION ALL SyntaxSELECT column_name(s) FROM table_name1 UNION ALL SELECTcolumn_name(s) FROM table_name2


9/13

Table Name: "Employees_Norway":

E_IDE_Name01Hansen, Ola

02Svendson, Tove03

Svendson, Stephen04Pettersen, KariTable Name: "Employees_USA":

Table Name: "Employees_Norway":

E_ID E_Name

01 Hansen, Ola

02 Svendson, Tove

03 Svendson, Stephen

04 Pettersen, Kari

Table Name: "Employees_USA":

E_ID E_Name

01 Turner, Sally

02 Kent, Clark

03 Svendson, Stephen

04 Scott, Stephen

SELECT E_Name FROM

Employees_Norway UNION

SELECT E_Name FROM

Employees_USA.

If we want to list all the different employees in Norway and USA.

Hansen, Ola

Svendson, Tove

Svendson, Stephen

Pettersen, Kari

Turner, Sally

Kent, Clark

Scott, Stephen

Note: This command cannotbe used to list all employeesin Norway and USA. In theexample above we have two

employees with equal names,and only one of them will belisted. The UNIONcommand selects onlydistinct values.


10/13

SQL UNION ALL

Now we want to list all employees in Norway and USA:

SELECT E_Name FROM Employees_Norway UNION ALL SELECT E_NameFROM Employees_USA

Result:

E_Name

Hansen, Ola

Svendson, Tove

Svendson, Stephen

Pettersen, Kari

Turner, Sally

Kent, Clark

Svendson, Stephen

Scott, Stephen


11/13

The SQL SELECT INTO Statement

The SELECT INTO statement selects data fromone table and inserts it into a different table.The SELECT INTO statement is most often usedto create backup copies of tables.

SELECT * INTO new_table_name [INexternaldatabase] FROM old_tablename


12/13

The SQL SELECT INTO Statement

Example: Make a Backup Copy.SELECT * INTO Persons_Backup FROM Persons

We can also copy only a few fields into the newtable:

SELECT LastName,FirstName INTO

Persons_Backup FROM Persons


13/13

SQl SELECT INTO statement

SQL SELECT INTO - With a WHERE ClauseThe following SQL statement creates a"Persons_Backup" table with only the personswho lives in the city "Sandnes":

SELECT LastName,Firstname INTOPersons_Backup FROM Persons WHERECity='Sandnes'

SQL SELECT INTO - Joined Tables

The following example creates a"Persons_Order_Backup" table contains datafrom the two tables "Persons" and "Orders":

Documents

SQL Joining