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Defining Inner Joins
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Inner joins return only the rows that match in the join condition. Although an inner join can be specified in either the FROM or the WHERE clause, it is recommended to specify the JOIN in the FROM clause. All samples and practices in this book follow this recommendation. The following sample returns the employee s first and last name from the Person.Person table and their job title, sick hours, and vacation hours from the HumanResources.Employee table. Aliases are not used in this sample. Notice that the columns in the select list do not
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require qualification because they are unique across both tables. Because the column name BusinessEntityID is used in both tables, it must be qualified with a table name each time it is referenced:
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SELECT FirstName, LastName, JobTitle, VacationHours, SickLeaveHours FROM HumanResources.Employee INNER JOIN Person.Person ON HumanResources.Employee.BusinessEntityID = Person.Person.BusinessEntityID;
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Because an inner join was defined, rows from the HumanResources.Employee table that do not have a matching row in the Person.Person table are not returned. The reverse is also true, in that rows from the Person.Person table that are not matched in the HumanResources .Employee table are not returned. The way to overcome this behavior is to use an OUTER JOIN operator.
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Defining Outer Joins
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An outer join can be used to return all rows from one table and only information from rows that are in common from the other table, or it can return all rows from all tables in the JOIN clause. The word OUTER can be omitted from the syntax, but you must specify LEFT, RIGHT, or FULL. The LEFT and RIGHT operators can be used to specify from which table all the rows are returned. When you specify LEFT OUTER JOIN, all rows are returned from the table to the left of the keyword JOIN. This table is referred to as the outer table. You can accomplish the same thing with either a LEFT or RIGHT operator by changing the order in which the table names are referenced. For example, the following sample code returns all rows from the Person.Person table, along with the corresponding information from the matching rows in the HumanResources.Employee table. Figure 1-5 shows the result set from this query. Notice the NULL values in the JobTitle, VacationHours, and SickLeaveHours columns from the HumanResources.Employee table. These values are not available for rows in the Person.Person table that do not have corresponding information in the HumanResources.Employee table. Because the HumanResources.Employee table does not allow NULL values for these columns, it is obvious which rows were added to the result set by changing the INNER JOIN to a RIGHT OUTER JOIN. If NULL values were allowed, you would not be able to use the presence of NULL values alone to determine which rows were added by changing the join type:
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SELECT FirstName, LastName, JobTitle, VacationHours, SickLeaveHours FROM HumanResources.Employee E RIGHT OUTER JOIN Person.Person P ON E.BusinessEntityID = P.BusinessEntityID;
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The same result set can be achieved by using a LEFT OUTER JOIN by reordering the table names, as in the following query:
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SELECT FirstName, LastName, JobTitle, VacationHours, SickLeaveHours FROM Person.Person P LEFT OUTER JOIN HumanResources.Employee E
ON E.BusinessEntityID = P.BusinessEntityID;
Lesson 2: Joining Related Tables
FiGURe 1-5 RIGHT OUTER result set
A FULL OUTER JOIN displays every row from every table in the JOIN clause. This can be helpful in finding unmatched rows when relational integrity is not being enforced on the tables. If there are foreign key constraints on the join column, a FULL OUTER JOIN provides the same result set as a LEFT OUTER JOIN with the table where the foreign key is defined on the left side of the JOIN keyword. This is because every value in the foreign key column must have a matching row in the primary key column for the constraint to be satisfied. In addition, when foreign key constraints are defined, an OUTER JOIN defined with the primary key table being defined as the outer table provides the same results as an INNER JOIN.
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