SQL For Dummies®

Chapter 9. Zeroing In on the Data You Want

In This Chapter

• Specifying the tables you want to work with

• Separating rows of interest from the rest

• Building effective WHERE clauses

• Handling null values

• Building compound expressions with logical connectives

• Grouping query output by column

• Putting query output in order

A database management system has two main functions: storing data and providing easy access to that data. Storing data is nothing special; a file cabinet can perform that chore. The hard part of data management is providing easy access. For data to be useful, you must be able to separate the (usually) small amount you want from the huge amount you don't want.

SQL enables you to use some characteristics of the data to determine whether a particular table row is of interest to you. The SELECT, DELETE, and UPDATE statements convey to the database engine (the part of the DBMS that directly interacts with the data) which rows to select, delete, or update. You add modifying clauses to the SELECT, DELETE, and UPDATE statements to refine the search to your specifications.

Modifying Clauses

The modifying clauses available in SQL are FROM, WHERE, HAVING, GROUP BY, and ORDER BY. The FROM clause tells the database engine which table or tables to operate on. The WHERE and HAVING clauses specify a data characteristic that determines whether or not to include a particular row in the current operation. The GROUP BY and ORDER BY clauses specify how to display the retrieved rows. Table 9-1 provides ...