module 09 advance query techniques

8/9/2019 Module 09 Advance Query Techniques

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Module 9:Advanced QueryTechniques

Vidya Vrat Agarwal. | MCT, MCSD


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Overview

Introduction to Subqueries

Subqueries Vs. Joins

Parallel Subqueries

Correlated SubqueriesUsing the EXISTS and NOT EXISTS Keywords


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Subqueries are Nested Select Statements.Subqueries are Select.From.Where expression nested inside another

such expression.

USE pubs

SELECT pub_name

FROMpublishersWHERE pub_idIN

(SELECT pub_id

FROMtitles

WHERE type = 'business')

Inner Query

Inner Select

Outer Query

Outer Select

The Inner query evaluate first and then based on its reslut the outerquery will be executed. i.e., Bottom-to-Top


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Why to Use Subqueries

To break down a complex query into a series of logical steps

To answer a query that relies on the results of another query

The ability to usea Query within a Queryor Nested Queryis the original reason for the wordStructuredin the name

SQL ( Structured Query Language)


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select distinctpub_name,title

from publishers pJOINtitles t

on p.pub_id =t.pub_id

where t.type = 'business

One difference in using a join rather than a subquery is that the join lets you show columns from more than one table in theresult.

Why to Use Joins Rather Than Subqueries

SQL Server executes joins faster than subqueries


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Using Subqueries

Enclose Subqueries in Parentheses

Use Only One Expression or Column Name in theselect list, cant use select * in Subquery.

Use Subqueries in Place of an ExpressionRelational Database Engine processes the nested SQLstatement first and then result will be returned to theouter query.

Cannot Use Subqueries on Columns Containing Textand Image Data Types

No Limit to the Levels of Subqueries


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USE northwindSELECT orderid, customeridFROM orders

WHERE orderdate = (SELECT max( orderdate )FROM orders)

USE northwindSELECT orderid, customeridFROM orders

WHERE orderdate = (SELECT max( orderdate )FROM orders)

Subqueries Returning a Single Value

Subquery Replaces Expression in:

select list

WHERE clause introduced with = comparison operator


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USE northwindSELECT companynameFROM customersWHERE customerid IN

(SELECT customeridFROM ordersWHERE orderdate >'1/1/95')

USE northwindSELECT companyname

FROM customersWHERE customerid IN

(SELECT customeridFROM ordersWHERE orderdate >'1/1/95')

Subqueries Returning a List of Values

Subquery Replaces Expression in:WHERE clause introduced with the IN keyword

Server: Msg 512, Level 16, State 1, Line 2Subquery returned more than 1 value . This is not permitted when thesubquery follows =, !=, = or when the subquery is used as anexpression.


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Other Comparison Operator

USE northwindSELECT orderid, customerid

FROM orders

WHERE orderdate> (SELECT orderdate FROM orders)

Server: Msg 512, Level 16, State 1, Line 2

Subquery returned more than 1 value. This is not permitted

when the subquery follows =, !=, = or when thesubquery is used as an expression.


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Other Comparison Operator

USE northwindSELECT orderid, customerid

FROM orders

WHERE orderdate > Any or Some (SELECT orderdateFROM orders)

ANYor SOMEmust be greater thanat least one of the

values in the list of values returned by the subquery


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Subqueries in UPDATE

Subqueries can be nested in UPDATE statements.The following query doubles the price of all books published by New Moon

Books. The query updates the titles table; its subquery references thepublishers table.

UPDATE titles SET price = price * 2WHERE pub_id IN

(SELECT pub_id

FROM publishers

WHERE pub_name = 'New Moon Books')


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Subqueries in DELETE

DELETE sales

WHERE title_id IN

(SELECT title_id

FROM titles


Subqueries can be nested in DELETE statements .


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Paraller Subqueries

A Query having a Compound Where clause.

Select * from Distributer

Where discount >= ( Select Avg(discount)from distributor)

And Ltime > (select Avg(Ltime)from Distributor)

AndCredit < (select Avg ( Credit)from distributor where city-London )


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Correlated Subquery

Back to Step 1Back to Step 1

SELECT orderid, customeridFROM orders or1WHERE 20 < (SELECT quantity

FROM [order details] odWHERE or1.orderid = od.orderid

AND od.productid = 23)

SELECT orderid, customeridFROM orders or1WHERE 20 < (SELECT quantity

FROM [order details] odWHERE or1.orderid = od.orderid

AND od.productid = 23)

Outer query passescolumn values to the inner query

Outer query passescolumn values to the inner query

111

Inner query usesthat value to satisfy the inner query

Inner query usesthat value to satisfy the inner query

222

Inner query returnsa value back to the outer query

Inner query returnsa value back to the outer query

333

The process is repeated for the nextcolumn value of the outer query

The process is repeated for the nextcolumn value of the outer query

444


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Using the EXISTS and NOT EXISTS Keywords

Use with Correlated Subqueries

Determine Whether Data Exists in a List of Values

SQL Server Process

Outer query tests for the existence of rows

Inner query returns TRUE or FALSE

Data is not produced

USE northwindSELECT lastname, employeeidFROM employees eWHERE EXISTS (SELECT * FROM orders

WHERE e.employeeid = orders.employeeidAND orderdate = '9/5/97')

USE northwindSELECT lastname, employeeidFROM employees eWHERE EXISTS (SELECT * FROM orders

WHERE e.employeeid = orders.employeeidAND orderdate = '9/5/97')


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Check Your Understanding.


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Q.1 What is SubQuery ?


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Q.2. Why to use Subqueries ?


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Q.3. A Subquery can be a Union ?

1. True

2. False


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Q.4. Will this query produce result .?SELECT *

FROM publishers

WHERE pub_id IN

(SELECT pub_idFROM titles


1. Yes2. No


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Q.5. Is there any limit to nest the queries. ?

1. Yes

2. No


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Q.6. What is the Mistake in the following SQL Subquery.?

USE northwind

SELECT orderid, customeridFROM orders

WHERE orderdate > (SELECT orderdate FROM orders)


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Q.7. Does SQL Server executes Joins faster thanSubqueries.?

1. True

2. False


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Q.8.Subqueries can not be nested in updatestatements..?

1. True

2. False


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Q.9. What will be the sequence of Execution of this query.USE pubsSELECT au_lname, au_fname

FROM authors

WHERE au_idIN( SELECT au_idFROM titleauthor WHERE title_idIN

( SELECT title_idFROM titles

WHERE type = 'popular_comp'))


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Q.10. What is the difference between SubQuery andCorrelated Query.?


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Overview



Parallel Subqueries

Correlated SubqueriesUsing the EXISTS and NOT EXISTS Keywords


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A burning desire is

The starting point of

All accomplishment.

Thank You.

module 09 advance query techniques

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