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Regardless of the academic debates, NULL values are a well-entrenched part of the ANSI/ISO SQL standard and are supported in virtually all commercial SQL products. They also play an important, practical role in production of SQL databases. The special rules that apply to NULL values (and the cases where NULL values are handled inconsistently by various SQL products) are pointed out throughout this book.
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This chapter described the basic elements of SQL. The basic structure of SQL can be summarized as follows:
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SQL that is in common use includes about 30 statements, each consisting of a verb and one or more clauses. Each statement performs a single, specific function. SQL-based databases can store various types of data, including text, integers, decimal numbers, floating point numbers, and usually many more vendor-specific data types. SQL statements can include expressions that combine column names, constants, and built-in functions, using arithmetic and other vendor-specific operators. Variations in data types, constants, and built-in functions make portability of SQL statements more difficult than it may seem at first. NULL values provide a systematic way of handling missing or inapplicable data in the SQL.
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Simple Queries
n many ways, queries are the heart of SQL. The SELECT statement, which is used to express SQL queries, is the most powerful and complex of the SQL statements. Despite the many options afforded by the SELECT statement, it s possible to start simply and then work up to more complex queries. This chapter discusses the simplest SQL queries those that retrieve data from individual rows of a single table in the database. If you have not done so already, you will learn more if you create the sample database on your own system and try the queries for yourself as you read. Instructions for the sample database are in Appendix A.
The SELECT Statement
The SELECT statement retrieves data from a database and returns it to you in the form of query results. As a reminder, the exact format of the query results will vary from one SQL product to another. You have already seen many examples of the SELECT statement in the quick tour presented in 2. Here are several more sample queries that retrieve information about sales offices: List the sales offices with their targets and actual sales.
SELECT CITY, TARGET, SALES FROM OFFICES; CITY TARGET SALES ------------ ------------ -----------Denver $300,000.00 $186,042.00 New York $575,000.00 $692,637.00 Chicago $800,000.00 $735,042.00 Atlanta $350,000.00 $367,911.00 Los Angeles $725,000.00 $835,915.00
Part II:
Retrieving Data
List the Eastern region sales offices with their targets and sales.
SELECT CITY, TARGET, SALES FROM OFFICES WHERE REGION = 'Eastern'; CITY TARGET SALES ------------ ------------ -----------New York $575,000.00 $692,637.00 Chicago $800,000.00 $735,042.00 Atlanta $350,000.00 $367,911.00
List Eastern region sales offices whose sales exceed their targets, sorted in alphabetical order by city.
SELECT FROM WHERE AND ORDER CITY, TARGET, SALES OFFICES REGION = 'Eastern' SALES > TARGET BY CITY;
CITY TARGET SALES ------------ ------------ -----------Atlanta $350,000.00 $367,911.00 New York $575,000.00 $692,637.00
For simple queries, the English language request and the SQL SELECT statement are very similar. When the requests become more complex, more features of the SELECT statement must be used to specify the query precisely. Figure 6-1 shows the full form of the SELECT statement, which consists of six clauses. The SELECT and FROM clauses of the statement are required. The remaining four clauses are optional. You include them in a SELECT statement only when you want to use the functions they provide. The following list summarizes the function of each clause: The SELECT clause lists the data items to be retrieved by the SELECT statement. The items may be columns from the database, or columns to be calculated by SQL as it performs the query. The SELECT clause is described in the next section. The FROM clause lists the tables and views that contain the data to be retrieved by the query. (Views are discussed in detail in 14.) Queries that draw their data from a single table are described in this chapter. More complex queries that combine data from two or more tables are discussed in 7. The WHERE clause tells SQL to include only certain rows of data in the query results. A search condition is used to specify the desired rows. The basic uses of the WHERE clause are described in the Row Selection (WHERE Clause) section later in this chapter. Those that involve subqueries are discussed in 9. The GROUP BY clause specifies a summary query. Instead of producing one row of query results for each row of data in the database, a summary query groups together similar rows and then produces one summary row of query results for each group. Summary queries are described in 8.
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