oracle 减少对表的查询

在含有子查询的SQL语句中,要特别注意减少对表的查询.

  例如:

     低效

          SELECT TAB_NAME

          FROM TABLES

          WHERE TAB_NAME = ( SELECT TAB_NAME

                                FROM TAB_COLUMNS

                                WHERE VERSION = 604)

          AND　DB_VER= ( SELECT DB_VER

                           FROM TAB_COLUMNS

                           WHERE VERSION = 604)

     高效

          SELECT TAB_NAME

          FROM TABLES

          WHERE  (TAB_NAME,DB_VER)

 = ( SELECT TAB_NAME,DB_VER)

                   FROM TAB_COLUMNS

                   WHERE VERSION = 604)

     Update 多个Column 例子:

     低效:

           UPDATE EMP

           SET EMP_CAT = (SELECT MAX(CATEGORY) FROM EMP_CATEGORIES),

              SAL_RANGE = (SELECT MAX(SAL_RANGE) FROM EMP_CATEGORIES)

           WHERE EMP_DEPT = 0020;

     高效:

           UPDATE EMP

           SET (EMP_CAT, SAL_RANGE)

 = (SELECT MAX(CATEGORY) , MAX(SAL_RANGE)

 FROM EMP_CATEGORIES)

           WHERE EMP_DEPT = 0020;