In this session, you will learn to:Query data by using joins
Query data by using subqueries
Objectives
JoinsAllow to retrieve data from multiple tables
Can be of the following types:Inner join
Outer join
Cross join
Equi join
Self join
Querying Data by Using Joins
A B C B D E
A B C D E
INNER JOIN
Table X Table Y
COLUMNS COLUMNS
OUTPUT
COMMON ROWS
Using an Inner Join
Inner Join:Retrieves data from multiple tables by using a comparison operator on a common column
Retrieves only those rows that satisfy the join condition
Syntax:SELECT column_name, column_name [,column_name]FROM table1_name JOIN table2_name ON table1_name.ref_column_name join_operatortable2_name.ref_column_name
Let’s see how…
Using an Inner Join (Contd.)
Just a minute
Why do you need a table alias with the column name?
Answer:A table alias is required to uniquely identify columns in the SELECT query to avoid ambiguity that arises due to same column names in multiple tables.
Outer Join:Displays the result set containing all rows from one table and the matching rows from another table
Displays NULL for the columns of the related table where it does not find matching records
Is of three types:
Left outer join
Right outer join
Full outer join
Using an Outer Join
A B C B D E
A B C D E
LEFT OUTER JOIN
Table X Table Y
COLUMNS COLUMNS
OUTPUT
ALL ROWS FROM TABLE X AND COMMON ROWS FROM TABLE Y
Using an Outer Join (Contd.)
A B C B D E
A B C D E
RIGHT OUTER JOIN
Table X Table Y
COLUMNS COLUMNS
OUTPUT
ALL ROWS FROM TABLE Y AND COMMON ROWS FROM TABLE X
Using an Outer Join (Contd.)
Using an Outer Join (Contd.)
A B C B D E
A B C D E
FULL OUTER JOIN
Table X Table Y
COLUMNS COLUMNS
OUTPUT
ALL ROWS FROM TABLE Y AND TABLE Y AND COMMON ROWS ONLY ONCE
Syntax:SELECT column_name, column_name [,column_name]FROM table1_name [LEFT | RIGHT | FULL] OUTER JOIN table2_name ON table1_name.ref_column_name join_operator table2_name.ref_column_name
Let’s see how…
Using an Outer Join (Contd.)
Just a minute
When do you use the right outer join?
Answer:You can use the right outer join when you need all the records from the table at the right side of the outer join and only the matching records from the the table at the left side of the outer join.
A B C B D E
A B C D E
CROSS JOIN
Table X Table Y
COLUMNS COLUMNS
OUTPUT
n ROWSm ROWS
ALL ROWS (n X m)EACH ROW OF TABLE X JOINED WITH EACH ROW OF TABLE Y
Using a Cross Join
Cross Join:Displays each row from the first table joined with each row from the second table
Produces the result set as the number of rows in the first table multiplied by the number of rows in the second table
Let’s see how…
Using a Cross Join (Contd.)
A B C B D E
A B C D B E
EQUI JOIN
Table X Table Y
COLUMNS COLUMNS
OUTPUT
COMMON ROWS
Using an Equi Join
Equi Join:Is same as an inner join
Displays all the columns from both the tables
Displays redundant column data in the result set
Let’s see how…
Using an Equi Join (Contd.)
Just a minute
What is the difference between an equi and an inner join?
Answer:An equi join is used to retrieve all the columns from both the tables. An inner join is used to retrieve selected columns from tables.
Self Join:Is used when one row in a table correlates with other rows in the same table
Uses alias name to differentiate the two copies of the same table
Let’s see how…
Using a Self Join
Problem Statement:The HR manager of AdventureWorks, Inc. requires a report containing the following details:
Employee ID
Employee Name
Department Name
Date of Joining
Employee Address
How will you generate this report?
Demo: Using Joins
Solution:To solve the preceding problem, you need to perform the following tasks:
1. Identify the join.
2. Create a query based on joins.
3. Execute the query to verify the result.
Demo: Using Joins (Contd.)
Subqueries:Are used when the result set of one query needs to be taken as input for another query
Syntax:SELECT column, column [,column]FROM table_nameWHERE column = ( SELECT column FROM table_name [WHERE conditional_expression] )
Querying Data by Using Subqueries
IN keyword:Is used to retrieve rows in a subquery based on the match of values given in a list
Syntax:SELECT column, column [,column]FROM table_nameWHERE column [ NOT ] IN ( SELECT column FROM table_name [WHERE conditional_expression] )
Let’s see how…
Using the IN and EXISTS Keyword
EXISTS keyword:Is used to check the existence of the data and returns true or false
Syntax:SELECT column, column [,column]FROM table_nameWHERE EXISTS ( SELECT column FROM table_name [WHERE
conditional_expression] )
Let’s see how…
Using the IN and EXISTS Keyword (Contd.)
Comparison operators are modified using:ALL keyword that returns TRUE, if all the values retrieved by the subquery satisfy the comparison operator
ANY keyword that returns TRUE, if any value retrieved by the subquery satisfies the comparison operator
Let’s see how…
Using Modified Comparison Operators
Just a minute
What is the use of EXISTS keyword in a subquery?
Answer:The EXISTS keyword is used to check the existence of rows in the result set of an inner query according to the condition specified in the inner query.
Aggregate functions:Can be used in the subquery to generate aggregated values from the inner query
Let’s see how…
Using Aggregate Functions
Nested subqueries:Contain one or more subqueries
Can be used when the condition of a query is dependent on the result of another query
Let’s see how…
Using Nested Subqueries
Correlated subqueries:Can be defined as a query that depends on the outer query for its evaluation
Uses the WHERE clause to refer to the table specified in the FROM clause
Let’s see how…
Using Correlated Subqueries
Just a minute
Write a query to determine the Employee ID and the Department ID of all the employees whose Manager ID is 12.
Answer:SELECT EmployeeID, DepartmentID FROM HumanResources.EmployeeDepartmentHistory WHERE EmployeeID=(SELECT EmployeeID FROM HumanResources.Employee WHERE ManagerID=12)
Problem Statement:The management of AdventureWorks, Inc. is planning to revise the pay rate of the employees. For this, they want a report containing the EmployeeID and designation of those employees whose present pay rate is more than 40.
How will you generate this report?
Demo: Using Subqueries
Solution:To solve the preceding problem, you need to perform the following tasks:
1. Create a query.
2. Execute the query to verify the result.
Demo: Using Subqueries (Contd.)
In this session, you learned that:Joins and subqueries are used to retrieve data from multiple tables.
An inner join combines records from multiple tables by using a comparison operator on a common column.
A left outer join returns all the rows from the left table and the matching rows from the right table.
A right outer join returns all the rows from the right table and the matching rows from the left table.
A full outer join returns all the matching and non-matching rows from both the tables on which join is applied.
A cross join returns each row from the first table joined with each row from the second table.
An equi join is used to list all the columns from the joining tables.
Summary
The IN clause in a subquery returns zero or more values.
The EXISTS clause in a subquery returns data in terms of a TRUE or FALSE value.
The ALL and ANY keyword is used in a subquery to modify the existing comparison operator.
Aggregate functions can also be used in subqueries to generate aggregated values from the inner query.
Subqueries that contain one or more queries are specified as nested subqueries.
A correlated subquery can be defined as a query that depends on the outer query for its evaluation.
A self join correlates one row in a table with other rows in the same table.
Summary (Contd.)