The previous chapters explained how to create tables, how to fill them with data, and how to manipulate that data. Now we finally discuss how to retrieve the data out of the database.
The process of retrieving or the command to retrieve data from a database is called a query. In SQL the SELECT command is used to specify queries. The general syntax of the SELECT command is
SELECT select_list FROM table_expression [sort_specification]The following sections describe the details of the select list, the table expression, and the sort specification.
The simplest kind of query has the form
SELECT * FROM emp;
Assuming that there is a table called emp, this command would retrieve all rows and all columns from emp. (The method of retrieval depends on the client application. For example, the EDB-PSQL+ program will display an ASCII-art table on the screen, while client libraries will offer functions to extract individual values from the query result.) The select list specification * means all columns that the table expression happens to provide. A select list can also select a subset of the available columns or make calculations using the columns. For example, if emp has columns named empno, ename, and sal (and perhaps others) you can make the following query:
SELECT empno, ename, sal FROM emp;
See Section 6.3 for more details.
FROM emp is a particularly simple kind of table expression: it reads just one table. In general, table expressions can be complex constructs of base tables, joins, and subqueries.
But you can also omit the table expression entirely and use the SELECT command for computing a constant expression. For this purpose we SELECT from a dummy table called DUAL. DUAL is accessible to all users and contains one column DUMMY, and contains one row with a value X.
SELECT 3 * 4 FROM dual;
This is more useful if the expressions in the select list return varying results. For example, you could call a function this way:
SELECT sysdate FROM dual;