Joins (A JOIN clause is used to combine rows from two or more tables, based on a related column between them.).pptx
- 2. 2 PostgreSQL - JOINS • So far, you have learned how to select data from a table, choosing which columns and rows you want, and how to sort the result set in a particular order. • Thus far, our queries have only accessed one table at a time. • Queries can access multiple tables at once, or access the same table in such a way that multiple rows of the table are being processed at the same time. • A query that accesses multiple rows of the same or different tables at one time is called a join query.
- 3. 3 PostgreSQL – JOINS … • As an example, say you wish to list all the weather records together with the location of the associated city. • To do that, we need to compare the city column of each row of the weather table with the name column of all rows in the cities table, and select the pairs of rows where these values match. • Note: • This is only a conceptual model. • The join is usually performed in a more efficient manner than actually comparing each possible pair of rows, but this is invisible to the user.
- 4. 4 What are PostgreSQL Joins? • PostgreSQL JOINs are used for retrieving data from more than one tables. • With JOINs, it is possible for us to combine the SELECT and JOIN statements into a single statement. • A JOIN condition is added to the statement, and all rows that meet the conditions are returned. • The values from different tables are combined based on common columns. • The common column mostly is a primary key in the first table and a foreign key of the second table.
- 5. 5 Types of PostgreSQL JOIN • The PostgreSQL Joins clause is used to combine records from two or more tables in a database. • A JOIN is a means for combining fields from two tables by using values common to each. • Join Types in PostgreSQL are − 1) The CROSS JOIN 2) The INNER JOIN 3) The OUTER JOIN I. The LEFT OUTER JOIN II. The RIGHT OUTER JOIN III. The FULL OUTER JOIN
- 6. 6 Types of PostgreSQL JOIN ….
- 7. 7 Types of PostgreSQL JOIN …
- 8. 8 Types of PostgreSQL JOIN … • Before we proceed, let us consider two tables, COMPANY and DEPARTMENT. • We already have seen INSERT statements to populate COMPANY table. • So just let us assume the list of records available in COMPANY table −
- 9. 9 Types of PostgreSQL JOIN • Finally, we have the following list of records available in DEPARTMENT table −
- 10. 10 1. The CROSS JOIN • A CROSS JOIN matches every row of the first table with every row of the second table. • If the input tables have x and y columns, respectively, the resulting table will have x+y columns. • Because CROSS JOINs have the potential to generate extremely large tables, care must be taken to use them only when appropriate. • The following is the syntax of CROSS JOIN − • Based on the above tables, we can write a CROSS JOIN as follows − SELECT ... FROM table1 CROSS JOIN table2 ...
- 11. 11 1. The CROSS JOIN : Example 1 • The above given query will produce the following result −
- 12. 12 1. The CROSS JOIN : Example 2 • Sample table: foods • Sample table: company
- 13. 13 1. The CROSS JOIN : Example 2 … • To get item name and item unit columns from foods table and company name, company city columns from company table, after a CROSS JOINING with these mentioned tables, the following SQL statement can be used: • How cross joining happened into two tables SELECT foods.item_name,foods.item_unit, company.company_name,company.company_city FROM foods CROSS JOIN company;
- 14. 14 1. The CROSS JOIN : Example 2 … • Output:
- 15. 15 1. The CROSS JOIN : Example 2 … • More presentation of the said output:
- 16. 16 2. The INNER JOIN • A 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 table1 and table2 are combined into a result row. • An INNER JOIN is the most common type of join and is the default type of join.
- 17. 17 2. The INNER JOIN : Example 1 • The following is the syntax of INNER JOIN − • Based on the above tables, we can write an INNER JOIN as follows − • The above given query will produce the following result − SELECT table1.column1, table2.column2... FROM table1 INNER JOIN table2 ON table1.common_filed = table2.common_field; SELECT EMP_ID, NAME, DEPT FROM COMPANY INNER JOIN DEPARTMENT ON COMPANY.ID = DEPARTMENT.EMP_ID;
- 18. 18 2. The INNER JOIN : Example 2 • Sample table: foods • Sample table: company
- 19. 19 2. The INNER JOIN : Example 2 … • To join item name, item unit columns from foods table and company name, company city columns from company table, with the following condition - • company_id of foods and company table must be same, • the following SQL statement can be used : • SQL Code: SELECT foods.item_name,foods.item_unit, company.company_name,company.company_city FROM foods JOIN company ON foods.company_id =company.company_id;
- 20. 20 2. The INNER JOIN : Example 2 … • Output:
- 21. 21 2. The INNER JOIN : Example 2 … • Pictorial Presentation:
- 22. 22 2. The INNER JOIN : Example 3 • Here is two table tableX and tableY and they have no duplicate rows in each. • In tableX the values ( A,B) are unique and in tableY the values (E,F) are unique, but the values (C and D) are common in both the tables.
- 23. 23 2. The INNER JOIN : Example 3 … • Here is INNER JOIN Output: • Here only the matching of both tableX and tableY have appeared in the result set. SELECT * FROM tableX INNER JOIN tableY on tableX.X = tableY.Y;
- 24. 24 3. The LEFT OUTER JOIN • The OUTER JOIN is an extension of the INNER JOIN. • SQL standard defines three types of OUTER JOINs: LEFT, RIGHT, and FULL and PostgreSQL supports all of these. • In case of LEFT OUTER JOIN, an inner join is performed first. • Then, for each row in table T1 that does not satisfy the join condition with any row in table T2, a joined row is added with null values in columns of T2. • Thus, the joined table always has at least one row for each row in T1.
- 25. 25 3. The LEFT OUTER JOIN : Example 1 • The following is the syntax of LEFT OUTER JOIN − • Based on the above tables, we can write an inner join as follows − • The above given query will produce the following result − SELECT ... FROM table1 LEFT OUTER JOIN table2 ON conditional_expression ... SELECT EMP_ID, NAME, DEPT FROM COMPANY LEFT OUTER JOIN DEPARTMENT ON COMPANY.ID = DEPARTMENT.EMP_ID;
- 26. 26 3. The LEFT OUTER JOIN : Example 2 • Sample table: foods • Sample table: company
- 27. 27 3. The LEFT OUTER JOIN : Example 2 … • SQL Code: • Explanation: • This SQL statement would return all rows from the company table and only those rows from the foods table where the joined fields are equal and if the ON clause matches no records in the 'foods' table, the join will still return rows, but the NULL in each column of the right table. SELECT company.company_id,company.company_name, company.company_city,foods.company_id,foods.item_name FROM company LEFT JOIN foods ON company.company_id = foods.company_id;
- 28. 28 3. The LEFT OUTER JOIN : Example 2 … • Output:
- 29. 29 3. The LEFT OUTER JOIN : Example 2 … • Pictorial Presentation of the above example:
- 30. 30 3. The LEFT OUTER JOIN : Example 3 • Here is LEFT OUTER JOIN • Output: • Here, all the rows from tableX that is left side of JOIN clause and all the rows with NULL values for unmatched columns from tableY that is the right side of JOIN clause have appeared. SELECT * FROM tableX LEFT OUTER JOIN tableY ON tableX.X= tableY.Y
- 31. 31 4. The RIGHT OUTER JOIN • First, an inner join is performed. • Then, for each row in table T2 that does not satisfy the join condition with any row in table T1, a joined row is added with null values in columns of T1. • This is the converse of a left join; the result table will always have a row for each row in T2. • The following is the syntax of RIGHT OUTER JOIN − SELECT ... FROM table1 RIGHT OUTER JOIN table2 ON conditional_expression ...
- 32. 32 4. The RIGHT OUTER JOIN : Example 1 • Based on the above tables, we can write an inner join as follows − • The above given query will produce the following result − SELECT EMP_ID, NAME, DEPT FROM COMPANY RIGHT OUTER JOIN DEPARTMENT ON COMPANY.ID = DEPARTMENT.EMP_ID;
- 33. 33 4. The RIGHT OUTER JOIN : Example 2 • Sample table: foods • Sample table: company
- 34. 34 4. The RIGHT OUTER JOIN : Example 2 … • SQL Code: • Explanation: • This SQL statement would return all rows from the foods table and only those rows from the company table where the joined fields are equal and if the ON clause matches no records in the company table, the join will still return rows, but the NULL in each column of company table. SELECT company.company_id,company.company_name, company.company_city,foods.company_id,foods.item_name FROM company RIGHT JOIN foods ON company.company_id = foods.company_id;
- 35. 35 4. The RIGHT OUTER JOIN : Example 2 … • Output:
- 36. 36 4. The RIGHT OUTER JOIN : Example 2 … • Pictorial Presentation of the above example:
- 37. 37 4. The RIGHT OUTER JOIN : Example 3 • Here is RIGHT OUTER JOIN • Output: • Here, all the rows from tableY that is the right side of JOIN clause and all the rows with NULL values for unmatched columns from tableX that is left side of JOIN clause have appeared. SELECT * FROM tableX RIGHT OUTER JOIN tableY ON tableX.X= tableY.Y
- 38. 38 5. The FULL OUTER JOIN • In SQL the FULL OUTER JOIN combines the results of both left and right outer joins and returns all (matched or unmatched) rows from the tables on both sides of the join clause. • First, an inner join is performed. • Then, for each row in table T1 that does not satisfy the join condition with any row in table T2, a joined row is added with null values in columns of T2. • In addition, for each row of T2 that does not satisfy the join condition with any row in T1, a joined row with null values in the columns of T1 is added. • The following is the syntax of FULL OUTER JOIN − SELECT ... FROM table1 FULL OUTER JOIN table2 ON conditional_expression ...
- 39. 39 5. The FULL OUTER JOIN : Example 1 • Based on the above tables, we can write an inner join as follows − • The above given query will produce the following result − SELECT EMP_ID, NAME, DEPT FROM COMPANY FULL OUTER JOIN DEPARTMENT ON COMPANY.ID = DEPARTMENT.EMP_ID;
- 40. 40 5. The FULL OUTER JOIN : Example 2 • Sample table: foods • Sample table: company
- 41. 41 5. The FULL OUTER JOIN : Example 2 … • As we know the FULL OUTER JOIN is the combination of the results of both LEFT OUTER JOIN and RIGHT OUTER JOIN, so, here we are going to describe how FULL OUTER JOIN perform internally. • SQL Code & o/p (Both LEFT & RIGHT OUTER JOINS):
- 42. 42 5. The FULL OUTER JOIN : Example 2 … • Pictorial Presentation of the above example:
- 43. 43 5. The FULL OUTER JOIN : Example 2 … • SQL Code: • Output: SELECT a.company_id AS "a.ComID", a.company_name AS "C_Name", b.company_id AS "b.ComID", b.item_name AS "I_Name" FROM company a FULL OUTER JOIN foods b ON a.company_id = b.company_id;
- 44. 44 5. The FULL OUTER JOIN : Example 2 … • Output:
- 45. 45 5. The FULL OUTER JOIN : Example 3 • Here is FULL OUTER JOIN • Output: • Here, all the matching rows from tableX and tableY and all the unmatched rows with NULL values for both the tables have appeared. SELECT * FROM tableX FULL OUTER JOIN tableY ON tableX.X= tableY.Y