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Database Homework Help | Practice Set: 5(Join In SQL Server)



In this blog we will cover all SQL Server Join related topics with the help of some example, below the list of topics which we have learn in this:

  • Table Joins/Types of Joins

  • INNER Joins

  • Left Outer Join

  • Right Outer Join

  • Cross Join

  • SELF Joins

  • Multiple Table Joins

  • Joins with Aggregate Functions

  • Practice Queries


Table​ ​Joins/Types​ ​of​ ​Joins

By using joins, you can retrieve data from two or more tables based on logical relationships between the tables. Joins indicate how SQL statements should use data from one table to SELECT the rows in another table. A join condition defines the way two tables are related in a query by: ● Specifying the column from each table to be used for the join. A typical join condition specifies a foreign key from one table and its associated key in the other table. ● Specifying a logical operator (for example, = or <>,) to be used in comparing VALUES from the columns.


INNER​ ​Joins

An inner join is a join in which the VALUES in the columns being joined are compared using a comparison operator. This inner join is known as an equi-join. It returns all the columns in both tables, and returns only the rows for which there is an equal VALUE in the join column. Inner joins eliminate the rows that do not match with a row from the other table. In the SQL-92 standard, inner joins can be specified in either the FROM or WHERE clause. This is the only type of join that SQL-92 supports in the WHERE clause. Inner joins specified in the WHERE clause are known as old-style inner joins.


EXAMPLE: The following SELECT statement returns the ProductID and Name from the Product table. This is a simple SELECT statement:


Write​ ​Exercise​ ​Code:

SELECT 
    ProductID, Name 
FROM 
    Product 


The following SELECT statement is an example of using INNER JOIN in SELECT statement and returns all data from both Employer and Contact tables that match ContactID. This is because we did not specify field names and used the asters * which brings back all fields.,


Write​ ​Exercise​ ​Code:

SELECT * FROM Employee AS e 
INNER JOIN Contact AS c 
ON e.ContactID = c.ContactID 
ORDER BY c.LastName 

Result

EmployeeID NationalIDNumber ContactID LoginID ManagerID

288 481044938 1012 adventure-works\syed0 273

235 6298838 1212 adventure-works\kim1 16 200 398223854 1268 adventure-works\hazem0 148 (Not all columns and rows represented)



The following SELECT statement modifies first SELECT statement and uses an INNER JOIN to return all data from both ProductVendor and Product tables that match ProductID: Write​ ​


Exercise​ ​Code:

SELECT 
ProductVendor.VendorID, Product.ProductID, Name 
FROM ProductVendor 
INNER JOIN Product 
ON ProductVendor.ProductID = Product.ProductID 

Result

In more common terms, this request could be similar to. “Can you pull a Vendor report that lists the Vendor ID along with the product IDs for products we carry?”


VendorID ProductID Name

83 1 Adjustable Race

57 2 Bearing Ball

85 4 Headset Ball Bearings

(not all rows represented)


The following SELECT statement modifies first SELECT statement and uses an INNER join to return all data from both ProductVendor and Product tables that match ProductID. It also only returns the top 10 records and includes an ORDER BY clause


Write​ ​Exercise​ ​Code:

SELECT Top 10 ProductVendor.VendorID, Product.ProductID, Name 
FROM ProductVendor 
INNER JOIN Product 
ON ProductVendor.ProductID = Product.ProductID 
ORDER BY VendorID

Result

VendorID ProductID Name

1 462 Lower Head Race

2 510 LL Road Rim

2 511 ML Road Rim

(Not all rows represented)


In this example we are joining between the SalesOrderDetail and Product tables. The tables are aliased with the following: SOD for SalesOrderDetail and P for Product. The JOIN logic is based on matching records in the SOD.ProductID and P.ProductID columns. The records are filtered by only returning records with the SOD.UnitPrice greater than 1000. Finally, the result set is returned in order with the most expensive first based on the ORDER BY clause and only the highest 100 products based on the TOP clause.


Write​ ​Exercise​ ​Code:

SELECT TOP 100 
P.ProductID, 
P.Name, 
P.ListPrice, 
P.Size, 
P.ModifiedDate, 
SOD.UnitPrice, 
SOD.UnitPriceDiscount, 
SOD.OrderQty, 
SOD.LineTotal 
FROM SalesOrderDetail SOD 
INNER JOIN Product P ON SOD.ProductID = P.ProductID 
WHERE SOD.UnitPrice > 1000 
ORDER BY SOD.UnitPrice DESC 

In more common terms, this request could be similar to. “Can you run a Sales Order Detail report where the Unit Price is great than $1000? I’d like to see the Unit Price (have it Sorted highest to lowest), and also show Discount, Order Quantity, and the Line total. Oh, also add the Product list price, size and date last modified to each product in the order detail report “


Result

ProductID Name ListPrice Size ModifiedDate

4749 Road-150 Red,62 3578.2700 62 2004-03-11 10:01:36.827

751 Road-150 Red, 48 3578.2700 48 2004-03-11 10:01:36.827

751 Road-150 Red, 48 3578.2700 48 2004-03-11 10:01:36.827

753 Road-150 Red, 56 3578.2700 56 2004-03-11 10:01:36.827

(Not all columns and rows represented)


The following SELECT statement is an example of using Implicit JOIN in SELECT statement and returns all data from both Productvendor and Product tables that match ProductID:


Write​ ​Exercise​ ​Code:

SELECT ProductVendor.VendorID, Product.ProductID, Product.Name 
FROM ProductVendor, Product 
WHERE ProductVendor.ProductID = Product.ProductID

Result

VendorID ProductID Name

83 1 Adjustable Race

57 2 Bearing Ball

85 4 Headset Ball Bearings

50 317 LL Crankarm

84 317 LL Crankarm

50 318 ML Crankarm

(Not all columns and rows represented)



Left​ ​Outer​ ​Join

Inner joins eliminate the rows that do not match with a row from the other table. Outer joins, however, return all rows from at least one of the tables or views mentioned in the FROM clause, as long as those rows meet any WHERE or HAVING search conditions. All rows are retrieved from the left table referenced with a left outer join, and all rows from the right table referenced in a right outer join. EXAMPLE: To include all contacts regardless if they made an order, use a left outer join. Here is the SQL query and results of the left outer join:


Write​ ​Exercise​ ​Code:

SELECT Contact.LastName, SalesOrderHeader.SalesOrderID 
FROM Contact Left Outer Join SalesOrderHeader 
ON Contact.ContactID = SalesOrderHeader.ContactID 
ORDER By Contact.LastName ASC 

Result

Note: the NULL field with Abbas. This implies Abbas doesn’t have any SalesOrder IDs attached to their contact.


LastName SalesOrderID

Abbas NULL

Abel 53459

Abel 71782

Abel 58907

Abel 65157

Abercrombie 44110

(Not all rows and columns displayed)


Right​ ​Outer​ ​Join

EXAMPLE: Based on the two tables specified in the join clause, all data is returned from the right table. On the left table, the matching data is returned in addition to NULL values where a record exists in the right table but not in the left table. 6To include all sales persons in the results, regardless of whether they are assigned a territory, use a right outer join. Here is the Transact-SQL query and results of the right outer join:


Write​ ​Exercise​ ​Code

SELECT st.Name AS Territory, sp.SalesPersonID 
FROM SalesTerritory st 
RIGHT OUTER JOIN SalesPerson sp 
ON st.TerritoryID = sp.TerritoryID ;

Results

Territory SalesPersonID

NULL 268

Northeast 275

Southwest 276

Central 277

Canada 278

Southeast 279

Northwest 280

(Not all rows displayed)


Cross​ ​Join

Cross join is a Cartesian join means Cartesian product of both the tables. This join does not need any condition to join two tables. This join returns records/rows that are multiplication of record number from both the tables means each row on left table will related to each row of right table. The result set contains 170 rows (SalesPerson has 17 rows and SalesTerritory has 10; 17 multiplied by 10 equals 170)


Write​ ​Exercise​ ​Code:

SELECT p.SalesPersonID, t.Name AS Territory 
FROM SalesPerson p 
CROSS JOIN SalesTerritory t 
ORDER BY p.SalesPersonID; 

Results

SalesPersonID Territory

268 Northwest

268 Northeast

268 Central

268 Southwest

268 Southeast

268 Canada

(Not all rows displayed)


SELF​ ​Joins A table can be joined to itself in a self-join. For example, you can use a self-join to find the Employee and the Manager. Because this query involves a join of the Employee table with itself, the Employee table appears in two roles. To distinguish these roles, you must give the Employee table two different aliases (e1 and e2) in the FROM clause. These aliases are used to qualify the column names in the rest of the query. EXAMPLE: This is an example of the self-join statement: Write​ ​Exercise​ ​Code:

SELECT e1.ManagerID AS ManagerID, e2.EmployeeID EmployeeID 
FROM Employee e1 
INNER JOIN Employee e2 ON e2.ManagerID = e1.EmployeeID 
ORDER BY e1.ManagerID

Result

ManagerID EmployeeID

NULL 6

NULL 12

NULL 42

NULL 140

NULL 148

NULL 273

3 79

3 114



Now let’s add the employee


Title Write​ ​Exercise​ ​Code:

SELECT e1.EmployeeID as ManagerID, e1.Title as ManagerTitle, e2.EmployeeID, e2.Title as EmployeeTitle 
FROM Employee e1 
JOIN Employee e2 ON e2.ManagerID = e1.EmployeeID 
WHERE ORDER BY e1.ManagerID 

Result

In more common terms, this request could be similar to. “Can you run a report for me on all Employees (listing their employee ID, Title), and their manager’s ID and title.”


ManagerID ManagerTitle EmployeeID EmployeeTitle

109 Chief Executive Officer 12 Vice President of Engineering 109 Chief Executive Officer 6 Marketing Manager

109 Chief Executive Officer 42 Information Services Manager 109 Chief Executive Officer 140 Chief Financial Officer



Multiple​ ​Table​ ​Joins

Although each join specification joins only two tables, the FROM clauses can contain multiple join specifications. This allows many tables to be joined for a single query. EXAMPLE: The ProductVendor table offers a good example of a situation in which joining more than two tables is helpful. The following SQL query finds the names of all products of a particular subcategory and the names of their vendors where ProductSubCategoryID = 15:


Write​ ​Exercise​ ​Code:

SELECT p.Name As ProductName, v.Name AS VendorName 
FROM Product p 
INNER JOIN ProductVendor pv ON p.ProductID = pv.ProductID 
INNER JOIN Vendor v ON pv.VendorID = v.VendorID 
WHERE ProductSubcategoryID = 15 
ORDER BY v.Name

Result

ProductName VendorName

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

LL Mountain Seat/Saddle Chicago City Saddles

ML Mountain Seat/Saddle Chicago City Saddles

HL Mountain Seat/Saddle Chicago City Saddles

(Not all rows displayed)



Joins​ ​with​ ​Aggregate​ ​Functions

To retrieve a list of all customers and the number of orders that each has placed we can use a join with an aggregate function EXAMPLE:


Write Exercise Code:

Select c.ContactID, c.LastName, Count(s.SalesOrderID) AS OrderID 
FROM Contact c Inner Join SalesOrderHeader s ON c.ContactID = s.ContactID 
Group BY c.LastName, c.ContactID

Result

contactID LastName OrderID

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

1 Achong 7

2 Abel 4

3 Abercrombie 12

4 Acevedo 11

5 Ackerman 4

6 Adams 12

(Not all rows displayed)


Combined​ ​Queries

Some queries are combined using the UNION operator. The UNION combines the results of two or more queries INTO a single result set that includes all the rows that belong to all queries in the union. The UNION operation is different from using joins that combine columns from two tables.


The following are basic rules for combining the result sets of two queries by using UNION:

  • The number and the order of the columns must be the same in all queries.

  • The data types must be compatible.


EXAMPLE:

The first SELECT statement returns all products with a price of more than 42.2100 The second SELECT statement uses the IN to find all products made by vendors 1001 and 1002. By default the UNION operator removes duplicates. To add duplicates just add the word ALL after UNION To combine these two statements do the following. First write the first SELECT statement and execute:


Write​ ​Exercise​ ​Code:

SELECT v.ProductID, v.VendorID, v.StandardPrice 
FROM ProductVendor v 
Where v.StandardPrice <= 42.2100


Then write second SELECT statement and execute:


Write Exercise Code:

SELECT v.ProductID, v.VendorID, v.StandardPrice 
FROM ProductVendor v 
Where v.VendorID IN (50,83)

When you combine the two statements using the UNION operator you basically are removing any duplicate records. To sort when you use the UNION operator you have to add only one ORDER BY clause at the end of the second statement. Otherwise you will get an error. To combine these two statements do the following: Write Exercise Code:

SELECT v.ProductID, v.VendorID, v.StandardPrice 
FROM ProductVendor v Where v.StandardPrice <= 42.2100 

UNION SELECT v.ProductID, v.VendorID, v.StandardPrice 

FROM ProductVendor v Where v.VendorID IN (50,83) 
order by v.StandardPrice asc

Result

ProductID VendorID StandardPrice

325 15 0.2000

326 15 0.2000

873 6 0.8000

352 64 1.1000

355 17 1.3000



To return all records when you combine the two statements you use the UNION ALL operator. To combine these two statements do the following:


Write​ ​Exercise​ ​Code:

SELECT v.ProductID, v.VendorID, v.StandardPrice 
FROM ProductVendor v Where v.StandardPrice <= 42.2100 
UNION ALL SELECT v.ProductID, v.VendorID, v.StandardPrice 
FROM ProductVendor v Where v.VendorID IN (50,83) 
order by v.StandardPrice asc

Result

To write the same UNION query above using multiple WHERE conditions you write it as follows:


Write​ ​Exercise​ ​Code:

SELECT v.ProductID, v.VendorID, v.StandardPrice 
FROM ProductVendor v 
WHERE v.StandardPrice <= 42.2100 OR v.VendorID IN (50,83) 
ORDER BY v.StandardPrice asc 

Result

ProductID VendorID StandardPrice

325 15 0.2000

326 15 0.2000

873 6 0.8000

352 64 1.1000

355 17 1.3000

352 12 1.4000



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