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Join Fundamentals in SQL

Posted by Shashi Ray Blogs | SQL Dec 11, 2008
Join Fundamentals in SQL

Join Fundamentals


Join Fundamentals...

By using joins, you can retrieve data from two or more tables based on logical relationships between the tables. Joins indicate how Microsoft® SQL Server 2000 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 (=, <>, and so on) to be used in comparing values from the columns.
Joins can be specified in either the FROM or WHERE clauses. The join conditions combine with the WHERE and HAVING search conditions to control the rows that are selected from the base tables referenced in the FROM clause.


Specifying the join conditions in the FROM clause helps separate them from any other search conditions that may be specified in a WHERE clause, and is the recommended method for specifying joins. A simplified SQL-92 FROM clause join syntax is:


FROM first_table join_type second_table [ON (join_condition)]
join_type specifies what kind of join is performed: an inner, outer, or cross join. join_condition defines the predicate to be evaluated for each pair of joined rows. This is an example of a FROM clause join specification:


FROM Suppliers JOIN Products
    ON (Suppliers.SupplierID = Products.SupplierID)
This is a simple SELECT statement using this join:
SELECT ProductID,
      Suppliers.SupplierID,
      CompanyName
FROM Suppliers JOIN Products
    ON (Suppliers.SupplierID = Products.SupplierID)
WHERE UnitPrice > $10
 AND CompanyName LIKE N'F%'
GO
The select returns the product and supplier information for any combination of parts supplied by a company for which the company name starts with the letter F and the price of the product is more than $10.

Using Joins


Join conditions can be specified in either the FROM or WHERE clauses; specifying them in the FROM clause is recommended. WHERE and HAVING clauses can also contain search conditions to further filter the rows selected by the join conditions.


Joins can be categorized as:
Inner joins (the typical join operation, which uses some comparison operator like = or <>). These include equi-joins and natural joins.


Inner joins use a comparison operator to match rows from two tables based on the values in common columns from each table. For example, retrieving all rows where the student identification number is the same in both the students and courses tables.


Outer joins. Outer joins can be a left, a right, or full outer join.
Outer joins are specified with one of the following sets of keywords when they are specified in the FROM clause:


LEFT JOIN or LEFT OUTER JOIN
The result set of a left outer join includes all the rows from the left table specified in the LEFT OUTER clause, not just the ones in which the joined columns match. When a row in the left table has no matching rows in the right table, the associated result set row contains null values for all select list columns coming from the right table.


RIGHT JOIN or RIGHT OUTER JOIN.
A right outer join is the reverse of a left outer join. All rows from the right table are returned. Null values are returned for the left table any time a right table row has no matching row in the left table.


FULL JOIN or FULL OUTER JOIN.
A full outer join returns all rows in both the left and right tables. Any time a row has no match in the other table, the select list columns from the other table contain null values. When there is a match between the tables, the entire result set row contains data values from the base tables.


Cross joins.
Cross joins return all rows from the left table; each row from the left table is combined with all rows from the right table. Cross joins are also called Cartesian products.

 

Shashi Ray

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