Having the capability to select data from multiple tables is one of SQL's most powerful features. Without this capability, the entire relational database concept would not be feasible. Single-table queries are sometimes quite informative, but in the real world, the most practical queries are those whose data is acquired from multiple tables within the database.
A relational database is broken up into smaller, more manageable tables for simplicity and the sake of overall management ease. As tables are divided into smaller tables, the related tables are created with common columns -- primary keys. These keys are used to join related tables to one another.
A join combines two or more tables to retrieve data from multiple tables. You might ask why you should normalize tables if, in the end, you are only going to rejoin the tables to retrieve the data you want. You rarely select all data from all tables, so it is better to pick and choose according to the needs of each individual query. Although performance may suffer slightly due to a normalized database, overall coding and maintenance are much simpler.
While different implementations have many ways of joining tables, you concentrate on the most common joins in this lesson. The types of joins that you learn are:
Perhaps the most used and important of the joins is the EQUIJOIN, also referred
SELECT TABLE1.COLUMN1, TABLE2.COLUMN2... FROM TABLE1, TABLE2 [, TABLE3 ] WHERE TABLE1.COLUMN_NAME = TABLE2.COLUMN_NAME [ AND TABLE1.COLUMN_NAME = TABLE3.COLUMN_NAME ] Look at the following example: SELECT EMPLOYEE_TBL.EMP_ID, EMPLOYEE_PAY_TBL.DATE_HIRE FROM EMPLOYEE_TBL, EMPLOYEE_PAY_TBL WHERE EMPLOYEE_TBL.EMP_ID = EMPLOYEE_PAY_TBL.EMP_ID;
This SQL statement returns the employee identification and the employee's date of hire. The employee identification is selected from the EMPLOYEE_TBL (although it exists in both tables, you must specify one table), whereas the hire date is selected from the EMPLOYEE_PAY_TBL. Because the employee identification exists in both tables, both columns must be justified with the table name. By justifying the columns with the table names, you tell the database server where to get the data.
Data in the following example is selected from tables EMPLOYEE_TBL and EMPLOYEE_PAY_TBL tables because desired data resides in each of the two tables. An equality join is used.
SELECT EMPLOYEE_TBL.EMP_ID, EMPLOYEE_TBL.LAST_NAME, EMPLOYEE_PAY_TBL.POSITION FROM EMPLOYEE_TBL, EMPLOYEE_PAY_TBL WHERE EMPLOYEE_TBL.EMP_ID = EMPLOYEE_PAY_TBL.EMP_ID; EMP_ID LAST_NAM POSITION --------- -------- ------------- 311549902 STEPHENS MARKETING 442346889 PLEW TEAM LEADER 213764555 GLASS SALES MANAGER 313782439 GLASS SALESMAN 220984332 WALLACE SHIPPER 443679012 SPURGEON SHIPPER 6 rows selected.
Notice that each column in the SELECT clause is preceded by the associated table name in order to identify each column. This is called qualifying columns in a query. Qualifying columns is only necessary for columns that exist in more than one table referenced by a query. You usually qualify all columns for consistency and to avoid any questions when debugging or modifying SQL code.
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This was first published in January 2003