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Querying SQL Data
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employee made during a three-year period. The SELECT statement averages the values in three different columns, as shown in the following SELECT statement:
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SELECT AVG(YEAR_1999), AVG(YEAR_2000), AVG(YEAR_2001) FROM EMPLOYEE_COMMISSIONS;
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In this case, there are three expressions that are used for the <derived column> placeholder: AVG(YEAR_1999), AVG(YEAR_2000), and AVG(YEAR_2001). Notice that each derived column expression is separated by a comma, as would be the case if each value were simply a column name. The following example shows the same SELECT statement as in the preceding example, except that it uses only column names as the derived columns:
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SELECT YEAR_1999, YEAR_2000, YEAR_2001 FROM EMPLOYEE_COMMISSIONS;
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If you were to execute this SELECT statement, your query would return all the values in the three columns, rather than averaging those values. The SELECT clause also allows you to provide a column name for each derived column. To do this, add the AS keyword and the new column name after the derived column, as shown in the following example:
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SELECT AVG(YEAR_1999) AS AVERAGE_1999 FROM EMPLOYEE_COMMISSIONS;
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In this SELECT statement, the value that is returned from the YEAR_1999 column is placed in a column named AVERAGE_1999. This is the name of the column that s returned as part of a virtual table in the query results. If you don t specify an AS subclause, the column name in the virtual table is the same as the column name in the source table. If a column name cannot be inherited naturally (for example, when adding two column values together), you must use the AS subclause. Notice that in the previous examples the FROM clause is used to specify the table (EMPLOYEE_COMMISSIONS) that contains the columns referred to in the SELECT clause. The FROM clause includes the FROM keyword and one or more table references. If there are multiple table references, they must be separated using commas. In most cases, the table reference is either the name of a table or of joined tables, although it can also be a type of subquery. I discuss joined tables in 11 and subqueries in 12. For this chapter, the FROM clause is used primarily to reference table names, as I have defined the clause in the two previous examples (where <table reference> equals EMPLOYEE_COMMISSIONS). Together the SELECT clause and the FROM clause form the foundation for the SELECT statement, which can be as simple as querying every row and every column of a table, as shown in the following example:
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SELECT * FROM PERFORMERS;
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In this statement, I specify that every column the PERFORMERS table should be returned. In addition, every row will be returned because no other clauses have been specified.
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SQL: A Beginner s Guide
PERFORMER_ID: PERFORMER_NAME: PLACE_OF_BIRTH: INT VARCHAR(60) VARCHAR (60) 2001 2002 2003 2004 2005 2006 2007 2008 2009 Jennifer Warnes Joni Mitchell William Ackerman Kitaro Bing Crosby Patsy Cline Jose Carreras Luciano Pavarotti Placido Domingo Seattle, Washington, USA Fort MacLeod, Alberta, Canada Germany Toyohashi, Japan Tacoma, Washington, United States Winchester, Virginia, United States Barcelona, Spain Modena, Italy Madrid, Spain
Figure 7-1
The PERFORMER_ID, PERFORMER_NAME, and PLACE_OF_BIRTH columns of the PERFORMERS table
Let s take a closer look at this. The PERFORMERS table includes the PERFORMER_ID, PERFORMER_NAME, and PLACE_OF_BIRTH columns, as shown in Figure 7-1. If you execute the SELECT statement shown in the previous example, your query results would look similar to the following:
PERFORMER_ID -----------2001 2002 2003 2004 2005 2006 2007 2008 2009 PERFORMER_NAME -----------------Jennifer Warnes Joni Mitchell William Ackerman Kitaro Bing Crosby Patsy Cline Jose Carreras Luciano Pavarotti Placido Domingo PLACE_OF_BIRTH -----------------------------Seattle, Washington, USA Fort MacLeod, Alberta, Canada Germany Toyohashi, Japan Tacoma, Washington, USA Winchester, Virginia, USA Barcelona, Spain Modena, Italy Madrid, Spain
Notice that every row and every column is returned in the query results. If you use the asterisk in your SELECT clause, you do not have to specify the column names. Now suppose you want to return only the PERFORMER_NAME and PLACE_OF_BIRTH columns. You could modify your SELECT statement to look like the following:
SELECT PERFORMER_NAME AS NAME, PLACE_OF_BIRTH FROM PERFORMERS;
7:
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