SQL Joins and SQL Views

Transcription

1 SQL Joins and SQL Views There are different types of joins available in SQL: INNER JOIN: returns rows when there is a match in both tables. LEFT JOIN: returns all rows from the left table, even if there are no matches in the right table. RIGHT JOIN: returns all rows from the right table, even if there are no matches in the left table. FULL JOIN: returns rows when there is a match in one of the tables. SELF JOIN: is used to join a table to itself as if the table were two tables, temporarily renaming at least one table in the SQL statement. CARTESIAN JOIN: returns the Cartesian product of the sets of records from the two or more joined tables. SQL - INNER JOINS The most frequently used and important of the joins is the INNER JOIN. They are also referred to as an EQUIJOIN. The INNER JOIN creates a new result table by combining column values of two tables (table1 and table2) based upon the join-predicate. The query compares each row of table1 with each row of table2 to find all pairs of rows which satisfy the join-predicate. When the join-predicate is satisfied, column values for each matched pair of rows of A and B are combined into a result row. Syntax: The basic syntax of INNER JOIN is as follows: SELECT table1.column1, table2.column2... FROM table1 INNER JOIN table2 ON table1.common_field = table2.common_field; Example: 4 Chaitali 25 Mumbai (b) Another table is ORDERS as follows: 1 P a g e INd_rasN

2 OID DATE ID AMOUNT :00: :00: :00: :00: Now, let us join these two tables using INNER JOIN as follows: INNER JOIN ORDERS ON CUSTOMERS.ID = ORDERS.CUSTOMER_ID; ID NAME AMOUNT DATE 4 Chaitali :00:00 SQL - LEFT JOINS The SQL LEFT JOIN returns all rows from the left table, even if there are no matches in the right table. This means that if the ON clause matches 0 (zero) records in right table, the join will still return a row in the result, but with NULL in each column from right table. This means that a left join returns all the values from the left table, plus matched values from the right table or NULL in case of no matching join predicate. Syntax: The basic syntax of LEFT JOIN is as follows: SELECT table1.column1, table2.column2... FROM table1 LEFT JOIN table2 ON table1.common_field = table2.common_field; Here given condition could be any given expression based on your requirement. Example: 2 P a g e INd_rasN

3 4 Chaitali 25 Mumbai (b) Another table is ORDERS as follows: OID DATE CUSTOMER_ID AMOUNT :00: :00: :00: :00: Now, let us join these two tables using LEFT JOIN as follows: LEFT JOIN ORDERS ON CUSTOMERS.ID = ORDERS.CUSTOMER_ID; ID NAME AMOUNT DATE 1 Ramesh NULL NULL 4 Chaitali :00:00 5 Hardik NULL NULL 6 Komal NULL NULL 7 Muffy NULL NULL SQL - RIGHT JOINS The SQL RIGHT JOIN returns all rows from the right table, even if there are no matches in the left table. This means that if the ON clause matches 0 (zero) records in left table, the join will still return a row in the result, but with NULL in each column from left table. This means that a right join returns all the values from the right table, plus matched values from the left table or NULL in case of no matching join predicate. Syntax: The basic syntax of RIGHT JOIN is as follows: SELECT table1.column1, table2.column2... FROM table1 RIGHT JOIN table2 ON table1.common_field = table2.common_field; 3 P a g e INd_rasN

4 Example: 4 Chaitali 25 Mumbai (b) Another table is ORDERS as follows: OID DATE CUSTOMER_ID AMOUNT :00: :00: :00: :00: Now, let us join these two tables using RIGHT JOIN as follows: RIGHT JOIN ORDERS ON CUSTOMERS.ID = ORDERS.CUSTOMER_ID; ID NAME AMOUNT DATE 4 Chaitali :00:00 SQL - FULL JOINS The SQL FULL JOIN combines the results of both left and right outer joins. The joined table will contain all records from both tables, and fill in NULLs for missing matches on either side. 4 P a g e INd_rasN

5 Syntax: The basic syntax of FULL JOIN is as follows: SELECT table1.column1, table2.column2... FROM table1 FULL JOIN table2 ON table1.common_field = table2.common_field; Here given condition could be any given expression based on your requirement. Example: 4 Chaitali 25 Mumbai (b) Another table is ORDERS as follows: OID DATE CUSTOMER_ID AMOUNT :00: :00: :00: :00: Now, let us join these two tables using FULL JOIN as follows: FULL JOIN ORDERS ON CUSTOMERS.ID = ORDERS.CUSTOMER_ID; ID NAME AMOUNT DATE 1 Ramesh NULL NULL 4 Chaitali :00:00 5 Hardik NULL NULL 6 Komal NULL NULL 7 Muffy NULL NULL 5 P a g e INd_rasN

6 4 Chaitali :00:00 If your Database does not support FULL JOIN like MySQL does not support FULL JOIN, then you can use UNION ALL clause to combine two JOINS as follows: LEFT JOIN ORDERS ON CUSTOMERS.ID = ORDERS.CUSTOMER_ID UNION ALL SELECT ID, NAME, AMOUNT, DATE RIGHT JOIN ORDERS ON CUSTOMERS.ID = ORDERS.CUSTOMER_ID SQL - SELF JOINS The SQL SELF JOIN is used to join a table to itself as if the table were two tables, temporarily renaming at least one table in the SQL statement. Syntax: The basic syntax of SELF JOIN is as follows: SELECT a.column_name, b.column_name... FROM table1 a, table1 b WHERE a.common_field = b.common_field; Here, WHERE clause could be any given expression based on your requirement. Example: 4 Chaitali 25 Mumbai Now, let us join this table using SELF JOIN as follows: SQL> SELECT a.id, b.name, a.salary a, CUSTOMERS b WHERE a.salary < b.salary; ID NAME SALARY 6 P a g e INd_rasN

7 Ramesh kaushik Chaitali Chaitali Chaitali Chaitali Hardik Hardik Hardik Hardik Hardik Komal Komal Komal Muffy Muffy Muffy Muffy Muffy Muffy SQL - CARTESIAN or CROSS JOINS The CARTESIAN JOIN or CROSS JOIN returns the Cartesian product of the sets of records from the two or more joined tables. Thus, it equates to an inner join where the joincondition always evaluates to True or where the join-condition is absent from the statement. Syntax: The basic syntax of CARTESIAN JOIN or CROSS JOIN is as follows: SELECT table1.column1, table2.column2... FROM table1, table2 [, table3 ] Example: 4 Chaitali 25 Mumbai (b) Another table is ORDERS as follows: OID DATE CUSTOMER_ID AMOUNT :00: :00: P a g e INd_rasN

8 :00: :00: Now, let us join these two tables using INNER JOIN as follows:, ORDERS; ID NAME AMOUNT DATE 1 Ramesh :00:00 1 Ramesh :00:00 1 Ramesh :00:00 1 Ramesh :00:00 2 Khilan :00:00 2 Khilan :00:00 2 Khilan :00:00 3 kaushik :00:00 3 kaushik :00:00 4 Chaitali :00:00 4 Chaitali :00:00 4 Chaitali :00:00 4 Chaitali :00:00 5 Hardik :00:00 5 Hardik :00:00 5 Hardik :00:00 5 Hardik :00:00 6 Komal :00:00 6 Komal :00:00 6 Komal :00:00 6 Komal :00:00 7 Muffy :00:00 7 Muffy :00:00 7 Muffy :00:00 7 Muffy :00:00 SQL Views A view is a virtual table. This chapter shows how to create, update, and delete a view. 8 P a g e INd_rasN

9 SQL CREATE VIEW Statement In SQL, a view is a virtual table based on the result-set of an SQL statement. A view contains rows and columns, just like a real table. The fields in a view are fields from one or more real tables in the database. You can add SQL functions, WHERE, and JOIN statements to a view and present the data as if the data were coming from one single table. SQL CREATE VIEW Syntax CREATE VIEW view_name AS SELECT column_name(s) FROM table_name WHERE condition Note: A view always shows up-to-date data! The database engine recreates the data, using the view's SQL statement, every time a user queries a view. SQL CREATE VIEW Examples If you have the Northwind database you can see that it has several views installed by default. The view "Current Product List" lists all active products (products that are not discontinued) from the "Products" table. The view is created with the following SQL: CREATE VIEW [Current Product List] AS SELECT ProductID,ProductName FROM Products WHERE Discontinued=No We can query the view above as follows: SELECT * FROM [Current Product List] Another view in the Northwind sample database selects every product in the "Products" table with a unit price higher than the average unit price: CREATE VIEW [Products Above Average Price] AS SELECT ProductName,UnitPrice FROM Products WHERE UnitPrice>(SELECT AVG(UnitPrice) FROM Products) We can query the view above as follows: SELECT * FROM [Products Above Average Price] Another view in the Northwind database calculates the total sale for each category in Note that this view selects its data from another view called "Product Sales for 1997": 9 P a g e INd_rasN

10 CREATE VIEW [Category Sales For 1997] AS SELECT DISTINCT CategoryName,Sum(ProductSales) AS CategorySales FROM [Product Sales for 1997] GROUP BY CategoryName We can query the view above as follows: SELECT * FROM [Category Sales For 1997] We can also add a condition to the query. Now we want to see the total sale only for the category "Beverages": SELECT * FROM [Category Sales For 1997] WHERE CategoryName='Beverages' SQL Updating a View You can update a view by using the following syntax: SQL CREATE OR REPLACE VIEW Syntax CREATE OR REPLACE VIEW view_name AS SELECT column_name(s) FROM table_name WHERE condition Now we want to add the "Category" column to the "Current Product List" view. We will update the view with the following SQL: CREATE OR REPLACE VIEW [Current Product List] AS SELECT ProductID,ProductName,Category FROM Products WHERE Discontinued=No SQL Dropping a View You can delete a view with the DROP VIEW command. SQL DROP VIEW Syntax DROP VIEW view_name 10 P a g e INd_rasN