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SELECT statement syntax diagram
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Simple Queries
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The SELECT Clause
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The SELECT clause that begins each SELECT statement specifies the data items to be retrieved by the query. The items are usually specified by a select list, a list of select items separated by commas. Each select item in the list generates a single column of query results, in left-to-right order. A select item can be one of the following: I A column name, identifying a column from the table(s) named in the FROM clause. When a column name appears as a select item, SQL simply takes the value of that column from each row of the database table and places it in the corresponding row of query results. I A constant, specifying that the same constant value is to appear in every row of the query results. I A SQL expression, indicating that SQL must calculate the value to be placed into the query results, in the style specified by the expression. Each type of select item is described later in this chapter.
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RETRIEVING DATA
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The FROM Clause
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The FROM clause consists of the keyword FROM, followed by a list of table specifications separated by commas. Each table specification identifies a table containing data to be retrieved by the query. These tables are called the source tables of the query (and of the SELECT statement) because they are the source of all of the data in the query results. All of the queries in this chapter have a single source table, and every FROM clause contains a single table name.
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Query Results
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The result of a SQL query is always a table of data, just like the tables in the database. If you type a SELECT statement using interactive SQL, the DBMS displays the query results in tabular form on your computer screen. If a program sends a query to the DBMS using programmatic SQL, the table of query results is returned to the program. In either case, the query results always have the same tabular, row/column format as the actual tables in the database, as shown in Figure 6-2. Usually the query results will be a table with several columns and several rows. For example, this query produces a table of three columns (because it asks for three items of data) and ten rows (because there are ten salespeople): List the names, offices, and hire dates of all salespeople.
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SELECT NAME, REP_OFFICE, HIRE_DATE FROM SALESREPS
SQL: The Complete Reference
NAME REP_OFFICE HIRE_DATE -------------- ----------- ---------Bill Adams 13 12-FEB-88 Mary Jones 11 12-OCT-89 Sue Smith 21 10-DEC-86 Sam Clark 11 14-JUN-88 Bob Smith 12 19-MAY-87 Dan Roberts 12 20-OCT-86 Tom Snyder NULL 13-JAN-90 Larry Fitch 21 12-OCT-89 Paul Cruz 12 01-MAR-87 Nancy Angelli 22 14-NOV-88
In contrast, the following query produces a single row because only one salesperson has the requested employee number. Even though this single row of query results looks less tabular than the multirow results, SQL still considers it to be a table of three columns and one row.
Figure 6-2.
The tabular structure of SQL query results
6:
Simple Queries
What are the name, quota, and sales of employee number 107
SELECT NAME, QUOTA, SALES FROM SALESREPS WHERE EMPL_NUM = 107 NAME QUOTA SALES -------------- ------------ -----------Nancy Angelli $300,000.00 $186,042.00 RETRIEVING DATA
In some cases the query results can be a single value, as in the following example: What are the average sales of our salespeople
SELECT AVG(SALES) FROM SALESREPS AVG(SALES) -----------$289,353.20
These query results are still a table, although it s a very small one consisting of one column and one row. Finally, it s possible for a query to produce zero rows of query results, as in this example: List the name and hire date of anyone with sales over $500,000.
SELECT NAME, HIRE_DATE FROM SALESREPS WHERE SALES > 500000.00 NAME HIRE_DATE ------------ ----------
Even in this situation, the query results are still a table. This one is an empty table with two columns and zero rows. Note that SQL s support for missing data extends to query results as well. If a data item in the database has a NULL value, the NULL value appears in the query results when the data item is retrieved. For example, the SALESREPS table contains NULL values in its QUOTA and MANAGER columns. The next query returns these NULL values in the second and third columns of query results.
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