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The tabular structure of SQL query results
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In some cases the query results can be a single value, as in the following example: What are the average sales of our salespeople
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SELECT AVG(SALES) FROM SALESREPS; AVG(SALES) -----------$289,353.20
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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.
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SELECT NAME, HIRE_DATE FROM SALESREPS WHERE SALES > 500000.00; NAME HIRE_DATE ------------ ----------
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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
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Retrieving Data
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. Note that not all SQL products display NULL values in the same way Oracle and DB2, for example, display nothing when a NULL value is encountered. List the salespeople, their quotas, and their managers.
SELECT NAME, QUOTA, MANAGER FROM SALESREPS; NAME QUOTA MANAGER -------------- ------------ -------Bill Adams $350,000.00 104 Mary Jones $300,000.00 106 Sue Smith $350,000.00 108 Sam Clark $275,000.00 NULL Bob Smith $200,000.00 106 Dan Roberts $300,000.00 104 Tom Snyder NULL 101 Larry Fitch $350,000.00 106 Paul Cruz $275,000.00 104 Nancy Angelli $300,000.00 108
The fact that a SQL query always produces a table of data is very important. It means that the query results can be stored back into the database as a table. It means that the results of two similar queries can be combined to form a larger table of query results. Finally, it means that the query results can themselves be the target of further queries. A relational database s tabular structure thus has a very synergistic relationship with the relational query facilities of SQL. Tables can be queried, and queries produce tables.
Simple Queries
The simplest SQL queries request columns of data from a single table in the database. For example, this query requests three columns from the OFFICES table: List the location, region, and sales of each sales office.
SELECT CITY, REGION, SALES FROM OFFICES; CITY -----------Denver New York Chicago Atlanta Los Angeles REGION SALES -------- -----------Western $186,042.00 Eastern $692,637.00 Eastern $735,042.00 Eastern $367,911.00 Western $835,915.00
The SELECT statement for simple queries like this one includes only the two required clauses. The SELECT clause names the requested columns; the FROM clause names the table or view that contains them.
6:
Simple Queries
Conceptually, SQL processes the query by going through the table named in the FROM clause, one row at a time. For each row, SQL takes the values of the columns requested in the select list and produces a single row of query results. The query results thus contain one row of data for each row in the table.
Calculated Columns
In addition to columns whose values come directly from the database, a SQL query can include calculated columns whose values are calculated from the stored data values. To request a calculated column, you specify a SQL expression in the select list. As discussed in 5, SQL expressions can involve addition, subtraction, multiplication, and division. You can also use parentheses to build more complex expressions. Of course the columns referenced in an arithmetic expression must have a numeric type. If you try to add, subtract, multiply, or divide columns containing text data, SQL will report an error. This query shows a simple calculated column: List the city, region, and amount over/under target for each office.
SELECT CITY, REGION, (SALES - TARGET) FROM OFFICES; CITY -----------Denver New York Chicago Atlanta Los Angeles REGION (SALES-TARGET) -------- --------------Western -$113,958.00 Eastern $117,637.00 Eastern -$64,958.00 Eastern $17,911.00 Western $110,915.00
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