Jcat
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          Pseudo columns

          A pseudo column is an item of data which does not belong in any particular table but which can be treated as if it did. Any SELECT list of columns can include these pseudo columns.
          SYSDATE         current date and time
          
          ROWNUM          sequence number assigned to retrieved rows
          
          ROWID           unique identifier for a row
          
          UID             number associated with a user
          
          USER            userid of current user
          

          The DUAL table

          ORACLE provides a "dummy" table called DUAL containing one column and one row. It is useful for selecting miscellaneous information from ORACLE.

          Some uses for DUAL

          • Display today's date

            SELECT SYSDATE FROM dual;

          • Display the value of 356 divided by 17, to three decimal places

            SELECT ROUND(356 / 17, 3) FROM dual;

          • Display the current user name

            SELECT USER FROM dual;

          posted on 2006-08-18 08:50 Jcat 閱讀(240) 評論(0)  編輯  收藏 所屬分類: Database
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