SQL Retrieving Data from Multiple Tables

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1 The Islamic University of Gaza Faculty of Engineering Dept. of Computer Engineering Database Lab (ECOM 4113) Lab 5 SQL Retrieving Data from Multiple Tables Eng. Ibraheem Lubbad

2 An SQL JOIN clause is used to combine rows from two or more tables, views, or materialized views, based on a common field between them. The following list type of the SQL JOINs you can use: 1) Inner join Return all rows that match the condition in both the tables 2) Outer join LEFT (OUTER) JOIN: Return all the rows from the left table in conjunction with the matching rows from the right table. If there are no columns matching in the right table, it returns NULL values. RIGHT (OUTER) JOIN: Return all the rows from the right table in conjunction with the matching rows from the left table. If there are no columns matching in the left table, it returns NULL values. FULL (OUTER) JOIN: Combines LEFT OUTER JOIN and RIGHT OUTER JOIN. It returns row from either table when the conditions are met and returns NULL value when there is no match. 3) Natural join It is based on the two conditions: The JOIN is made on all the columns with the same name for quality Removes duplicate columns from the result. 4) Cross join It is the Cartesian product of the two tables involved. 5) Self It is a JOIN (INNER, OUTER, etc) of a table to itself.

3 Join Conditions: Natural: Creates an implicit join base on columns that have the same name in both tables, also common column must have the same datatype, otherwise return error. Using ( column_name) : Creates an explicit join base on columns that have the same name in both tables, usually use key attribute to join two table. On: creates an explicit join using join condition to bind to two table, use it when key attribute in each column has different name, also we can add conditional expression as where. Note: All INNER and OUTER keywords are optional Join Conditions (natural, using, on) can be combined with any join type In general, the syntax of an SQL JOIN clause is: Syntax of SQL Select Statement SELECT TABLE1. COLUMN, TABLE2. COLUMN FROM TABLE1 [NATURAL JOIN TABLE2] [JOIN TABLE2 USING (COLUMN_NAME)] [JOIN TABLE2 ON (TABLE1. COLUMN_NAME = TABLE2. COLUMN_NAME)] [LEFT RIGHT FULL OUTER JOIN TABLE2 ON (TABLE1. COLUMN_NAME = TABLE2. COLUMN_NAME)] [CROSS JOIN TABLE2];

4 INNER JOIN (INNER) JOIN: Return records that have matching values in both tables. Syntax of SQL Select Statement TABLE1 [INNER] JOIN TABLE2 USING (COLUMN_NAME) ON (TABLE1. COLUMN_NAME = TABLE2. COLUMN_NAME) ; Example: Find a list of all students, displaying their ID, and name, dept_name, and tot_cred, along with the courses that they have taken Use NATURAL clause SELECT ID,NAME,DEPT_NAME,TOT_CRED,COURSE_ID FROM STUDENT NATURAL JOIN TAKES ; Use USING clause SELECT ID, NAME,DEPT_NAME,TOT_CRED,COURSE_ID FROM STUDENT JOIN TAKES USING (ID) Use on clause SELECT STUDENT.ID,NAME,DEPT_NAME,TOT_CRED,COURSE_ID FROM STUDENT JOIN TAKES ON STUDENT.ID=TAKES.ID ;

5 OUTER JOIN By default, joining tables with the NATURAL JOIN, USING, or ON clauses results in an inner join. Any unmatched rows are not displayed in the output. To return the unmatched rows, you can use an outer join. An outer join returns all rows that satisfy the join condition and also returns some or all of those rows from one table for which no rows from the other table satisfy the join condition. If the SELECT statement in which the JOIN operations appears has an asterisk (*) in the select list, the asterisk will be expanded to the following list of columns (in this order): All the common columns. Every column in the first (left) table that is not a common column. Every column in the second (right) table that is not a common column There are three types of outer joins: 1. LEFT OUTER 2. RIGHT OUTER 3. FULL OUTER OUTER JOIN Syntax TABLE1 { LEFT RIGHT FULL} [OUTER] JOIN TABLE2 USING (COLUMN_NAME) ON (TABLE1. COLUMN_NAME = TABLE2. COLUMN_NAME) ;

6 1) Left OUTER JOIN: Example: Find INSTRUCTORs that do not work in any department SELECT * FROM INSTRUCTOR LEFT JOIN DEPARTMENT USING(DEPT_NAME); Note first column condition attribute Instructor table columns Department table columns Instructor ibraheem do not work in any department so all department attribute returned as null value. Also we can use natural condition to join between two table base common column tables. SELECT * FROM INSTRUCTOR NATURAL LEFT JOIN DEPARTMENT

7 2) Right OUTER JOIN: SELECT * FROM INSTRUCTOR RIGHT JOIN DEPARTMENT USING(DEPT_NAME); There isn t any instructor in the Department of Mathematics. 3) Full OUTER JOIN: Combines LEFT OUTER JOIN and RIGHT OUTER JOIN SELECT * FROM INSTRUCTOR FULL JOIN DEPARTMENT USING(DEPT_NAME); Left table Right table

8 Self-join: Joining a Table to Itself Example: Find the names of all instructors whose salary is greater than at least one instructor in the Biology department. SELECT DISTINCT T.NAME,T.SALARY,S.NAME, S.SALARY FROM INSTRUCTOR T JOIN INSTRUCTOR S ON T.ID <> S.ID WHERE T.SALARY > S.SALARY AND S.DEPT_NAME = 'Biology'; Join multiple tables: To execute a join of three or more tables, first joins two of the tables based on the join conditions comparing their columns and then joins the result to another table based on join conditions containing columns of the joined tables and the new table. Example: Find all instructors names and all courses name they taught SELECT NAME AS INSTRUCTOR_NAME, C.TITLE AS COURSE_NAME FROM INSTRUCTOR I JOIN TEACHES T ON I.ID=T.ID JOIN COURSE C ON T.COURSE_ID=C.COURSE_ID;

9 Join with Grouping: Apply aggregation function on table that containing columns of the joined tables Example: For each course section offered in 2009, find the average total credits ( tot cred) of all students enrolled in the section, which had at least 2 students SELECT COURSE_ID,SEC_ID,SEMESTER,YEAR,AVG(TOT_CRED) FROM TAKES NATURAL JOIN STUDENT WHERE YEAR=2009 GROUP BY COURSE_ID,SEC_ID,SEMESTER,YEAR HAVING COUNT (ID) >= 2; Example: Find the number of instructors in each department who teach a course in the Spring 2010 semester SELECT DEPT_NAME, COUNT (DISTINCT ID) AS INSTR_COUNT FROM INSTRUCTOR NATURAL JOIN TEACHES WHERE SEMESTER = 'Spring' AND YEAR = 2010 GROUP BY DEPT_NAME; END