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Instead of column names, you can also specify column expressions in the SELECT clause. For example, Listing 4-5 shows how you can derive the range of the salary grades in the SALGRADES table, by selecting the difference between upper limits and lower limits. Listing 4-5. Using a Simple Expression in a SELECT Clause select grade, upperlimit - lowerlimit from salgrades; GRADE UPPERLIMIT-LOWERLIMIT -------- --------------------1 500 2 199
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In the next example, shown in Listing 4-6, we concatenate the employee names with their initials into a single column, and also calculate the yearly salary by multiplying the monthly salary with 12. Listing 4-6. Another Example of Using Expressions in a SELECT Clause select , from where init||' '||ename name 12 * msal yearsal employees deptno = 10;
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NAME YEARSAL -------------------------------- -------AB CLARK 29400 CC KING 60000 TJA MILLER 15600 Now take a look at the rather odd query shown in Listing 4-7. Listing 4-7. Selecting an Expression with Literals select 3 + 4 from departments; 3+4 -------7 7 7 7 The query result might look strange at first; however, it makes sense when you think about it. The outcome of the expression 3+4 is calculated for each row of the DEPARTMENTS table. This is done four times, because there are four departments and we did not specify a WHERE clause. Because the expression 3+4 does not contain any variables, the result (7) is obviously the same for every department row.
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It makes more sense to execute queries such as the one shown in Listing 4-7 against a dummy table, with only one row and one column. You could create such a table yourself, but the Oracle DBMS supplies a standard dummy table for this purpose, named DUAL, which is stored in the data dictionary. Because the Oracle DBMS knows that the DUAL table contains only one single row, you usually get better performance results by using the DUAL table rather than a dummy table that you created yourself.
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Tip In 10g and above, the Oracle DBMS treats the use of DUAL like a function call that simply evaluates the expression used in the column list. This provides even better performance results than directly accessing the DUAL table.
Listing 4-8 shows two examples of DUAL table usage. Note that the contents of this DUAL table are totally irrelevant; you use only the property that the DUAL table contains a single row. Listing 4-8. Using the DUAL Table select 123 * 456 from dual; 123*456 -------56088 select sysdate from dual; SYSDATE ----------05-SEP-2004 The second query in Listing 4-8 shows an example of using the system date. You can refer to the system date in Oracle with the keyword SYSDATE. Actually, to be more precise, SYSDATE is a function that returns the system date. These functions are also referred to as pseudo columns. See Appendix A of this book for examples of other such pseudo columns. Listing 4-9 shows an example of using SYSDATE to derive the age of an employee, based on the date of birth stored in the BDATE column of the EMPLOYEES table. Listing 4-9. Using the System Date select ename, (sysdate-bdate)/365 from employees where empno = 7839; ENAME (SYSDATE-BDATE)/365 -------- ------------------KING 51.83758
Note The results of your queries using SYSDATE depend on the precise moment the command was run; therefore, when you execute the examples, the results will not be the same as those shown in Listings 4-8 and 4-9.
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Null Values in Expressions
You should always consider the possibility of null values occurring in expressions. In case one or more variables in an expression evaluate to a null value, the result of the expression as a whole becomes unknown. We will discuss this area of concern in more detail later in this chapter, in Section 4.9. As an appetizer, look at the result of the query in Listing 4-10. Listing 4-10. The Effect of Null Values in Expressions select ename, msal, comm, 12*msal + comm from employees where empno < 7600; ENAME MSAL COMM 12*MSAL+COMM -------- -------- -------- -----------SMITH 800 ALLEN 1600 300 19500 WARD 1250 500 15500 JONES 2975 As you can see, the total yearly salary (including commission) for two out of four employees is unknown, because the commission column of those employees contains a null value.
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