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n many ways, queries are the heart of the SQL language. 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 a single table in the database.
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The SELECT Statement
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The SELECT statement retrieves data from a database and returns it to you in the form of query results. 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.
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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
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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.
6:
Simple Queries
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
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What are the average target and sales for Eastern region offices
SELECT AVG(TARGET), AVG(SALES) FROM OFFICES WHERE REGION = 'Eastern' AVG(TARGET) AVG(SALES) ------------ -----------$575,000.00 $598,530.00
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: I 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. I The FROM clause lists the tables that contain the data to be retrieved by the query. 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. I 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.
SQL: The Complete Reference
I 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. I The HAVING clause tells SQL to include only certain groups produced by the GROUP BY clause in the query results. Like the WHERE clause, it uses a search condition to specify the desired groups. The HAVING clause is described in 8. I The ORDER BY clause sorts the query results based on the data in one or more columns. If it is omitted, the query results are not sorted. The ORDER BY clause is described in the Sorting Query Results (ORDER BY Clause) section later in this chapter.
Figure 6-1.
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