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RETRIEVAL: THE BASICS
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specifies the table name. The FROM clause becomes more interesting when you want to access multiple tables in a single query, as described in 8.
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4.2 The SELECT Clause
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Let s start with a straightforward example of a SELECT command, shown in Listing 4-1. Listing 4-1. Issuing a Simple SELECT Command select * from departments; DEPTNO -------10 20 30 40 DNAME ---------ACCOUNTING TRAINING SALES HR LOCATION MGR -------- -------NEW YORK 7782 DALLAS 7566 CHICAGO 7698 BOSTON 7839
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The asterisk (*) means to show all columns of the DEPARTMENTS table. Listing 4-2 shows a slightly more complicated query that selects specific columns from the EMPLOYEES table and uses a WHERE clause to specify a condition for the rows retrieved. Listing 4-2. Selecting Specific Columns select ename, init, job, msal from employees where deptno = 30; ENAME -------ALLEN WARD MARTIN BLAKE TURNER JONES INIT ----JAM TF P R JJ R JOB MSAL -------- -------SALESREP 1600 SALESREP 1250 SALESREP 1250 MANAGER 2850 SALESREP 1500 ADMIN 800
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Let s look at the syntax (the statement construction rules of a language) of this statement more closely. You have a lot of freedom in this area. For example, you can enter an entire SQL command in a single line, spread a SQL command over several lines, and use as many spaces and tabs as you like. New lines, spaces, and tabs are commonly referred to as white space. The amount of white space in your SQL statements is meaningless to the Oracle DBMS.
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Tip It is a good idea to define some SQL statement layout standards and stick to them. This increases both the readability and the maintainability of your SQL statements. At this point, our SQL statements are short and simple, but in real production database environments, SQL statements are sometimes several pages long.
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In the SELECT clause, white space is mandatory after the keyword SELECT. The columns (or column expressions) are separated by commas; therefore, white space is not mandatory. However, as you can see in Listing 4-2, spaces after the commas enhance readability. White space is also mandatory after the keywords FROM and WHERE. Again, any additional white space is not mandatory, but it might enhance readability. For example, you can use spaces around the equal sign in the WHERE clause.
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Column Aliases
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By default, the column names of the table are displayed above your query result. If you don t like those names for example, because they do not adequately describe the meaning of the column in the specific context of your query you can specify different result column headings. You include the heading you want to appear, called a column alias, in the SELECT clause of your query, as shown in the example in Listing 4-3. Listing 4-3. Changing Column Headings select ename, init, msal salary from employees where deptno = 30; ENAME -------ALLEN WARD MARTIN BLAKE TURNER JONES INIT SALARY ----- -------JAM 1600 TF 1250 P 1250 R 2850 JJ 1500 R 800
In this example, there is no comma between MSAL and SALARY. This small detail has a great effect, as the result in Listing 4-3 shows: SALARY is used instead of MSAL as a column heading (compare this with the result shown in Listing 4-2). By the way, the ANSI/ISO SQL standard also supports the optional keyword AS between any column name and its corresponding column heading (column alias). Using this keyword enhances readability. In other words, you can also formulate the query in Listing 4-3 as follows: select ename, init, msal AS salary from employees where deptno = 30;
RETRIEVAL: THE BASICS
The DISTINCT Keyword
Sometimes, your query results contain duplicate rows. You can eliminate such rows by adding the keyword DISTINCT immediately after the keyword SELECT, as demonstrated in Listing 4-4. Listing 4-4. Using DISTINCT to Eliminate Duplicate Rows select DISTINCT job, deptno from employees; JOB DEPTNO -------- -------ADMIN 10 ADMIN 30 DIRECTOR 10 MANAGER 10 MANAGER 20 MANAGER 30 SALESREP 30 TRAINER 20 8 rows selected. Without the addition of DISTINCT, this query would produce 14 rows, because the EMPLOYEES table contains 14 rows. Remove the keyword DISTINCT from the first line of the query in Listing 4-4, and then execute the query again to see the difference.
Note Using DISTINCT in the SELECT clause might incur some performance overhead, because the Oracle DBMS must sort the result in order to eliminate the duplicate rows.
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