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oracle函数之case和decode的用法区别及性能比较
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在oracle世界，你可以使用： 1）case表达式 或者 2）decode函数 来实现逻辑判断。Oracle的DECODE函数功能很强，灵活运用的话可以避免多次扫描，从而提高查询的性能。而CASE是9i以后提供的语法，这个语法更加的灵活，提供了IF THEN ELSE的功能。 case表达式 case表达式，可分两种，简单和搜索，简单case后接表达式，如：
对于简单的case需要几点注意： 1）寻找when的优先级：从上到下 2）再多的when，也只有一个出口，即其中有一个满足了expr就马上退出case 3）不能把return_expr和else_expr指定为null，而且，expr、comparison_expr和return_expr的数据类型必须相同。
搜索case： CASE WHEN condition THEN return_expr [WHEN condition THEN return_expr] ... ELSE else_expr END 例子： [sql] view plaincopySELECT (CASE WHEN cust_credit_limit BETWEEN 0 AND 3999 THEN ' 0 - 3999' WHEN cust_credit_limit BETWEEN 4000 AND 7999 THEN ' 4000 - 7999' WHEN cust_credit_limit BETWEEN 8000 AND 11999 THEN ' 8000 - 11999' WHEN cust_credit_limit BETWEEN 12000 AND 16000 THEN '12000 - 16000' END) AS BUCKET, COUNT(*) AS Count_in_Group FROM customers WHERE cust_city = 'Marshal' GROUP BY (CASE WHEN cust_credit_limit BETWEEN 0 AND 3999 THEN ' 0 - 3999' WHEN cust_credit_limit BETWEEN 4000 AND 7999 THEN ' 4000 - 7999' WHEN cust_credit_limit BETWEEN 8000 AND 11999 THEN ' 8000 - 11999' WHEN cust_credit_limit BETWEEN 12000 AND 16000 THEN '12000 - 16000' END); BUCKET COUNT_IN_GROUP ------------- -------------- 0 - 3999 8 4000 - 7999 7 8000 - 11999 7 12000 - 16000 1用decode可以违反第3NF（行不可再分，列不可再分，列不可重复）：列重复 [sql] view plaincopyhr@ORCL> select * from a; ID NAME ---------- ---------- 1 a 2 b 3 c 1 a hr@ORCL> select sum(decode(id,1,1,0)) think, 2 sum(decode(id,2,2,0)) water, 3 sum(decode(id,3,3,0)) linshuibin 4 from a; THINK WATER LINSHUIBIN ---------- ---------- ---------- 2 2 3
一个字段，decode函数可以完全改写简单case； 多个字段，需要复杂的case，方可。 语法： DECODE(value,if1,then1,if2,then2,if3,then3,...,else)，表示如果value等于if1时，DECODE函数的结果返then1,...,如果不等于任何一个if值，则返回else。可以用函数或表达式来替代value，if，then，else从而作出一些更有用的比较。 来看看具体的运用： 1 假设我们想给百度职员加工资，其标准是：工资在8000元以下的将加20％；工资在8000元以上的加15％ 则： select decode(sign(salary - 8000),1,salary*1.15,-1,salary*1.2,salary) "revised_salary" from employee 2 表table_subject，有subject_name列。要求按照：语、数、外的顺序进行排序 则： select * from table_subject order by decode(subject_name, '语文', 1, '数学', 2, , '外语',3) decode和简单case的性能比较
Oracle的DECODE函数功能很强，灵活运用的话可以避免多次扫描，从而提高查询的性能。而CASE是9i以后提供的语法，这个语法更加的灵活，提供了IF THEN ELSE的功能。 对于很多情况，DECODE和CASE都能解决问题，个人更倾向于使用DECODE，一方面是从8i保留下来的习惯，另一方面是DECODE的语法更加的简洁，代码量要小一些。 不过今天在看Oracle9i的数据仓库手册时发现，Oracle在文档中提到CASE语句的效率会更高一些，尤其是CASE表达式 WHEN 常量 THEN的语法，效率要比CASE WHEN表达式 THEN的语法更高一些。对于后面这种说法倒是没有太多的疑问，对于CASE比DECODE效率高这种说法倒是第一次看到，印象中DECODE效率很高，应该不会比CASE的效率差。 到底效率如何，还是要具体的实例来说： SQL> CREATE TABLE T AS2 SELECT A.*3 FROM DBA_OBJECTS A, DBA_MVIEWS; Table created. SQL> SELECT COUNT(*) FROM T; COUNT(*)----------6075760 下面检查DECODE和两种CASE语句的效率： SQL> SET ARRAY 1000SQL> SET TIMING ONSQL> SET AUTOT TRACE SQL> SELECT DECODE(OWNER, 'SYSTEM', 'SYSTEM', 'SYS', 'SYSTEM', 'USER') 2 FROM T; 6075760 rows selected. Elapsed: 00:00:07.24 Execution Plan----------------------------------------------------------Plan hash value: 1601196873 --------------------------------------------------------------------------| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |--------------------------------------------------------------------------| 0 | SELECT STATEMENT | | 4245K| 68M| 13828 (1)| 00:03:14 || 1 | TABLE ACCESS FULL| T | 4245K| 68M| 13828 (1)| 00:03:14 |-------------------------------------------------------------------------- Note------ dynamic sampling used for this statement Statistics----------------------------------------------------------0 recursive calls0 db block gets47551 consistent gets0 physical reads0 redo size46288564 bytes sent via SQL*Net to client67317 bytes received via SQL*Net from client6077 SQL*Net roundtrips to/from client0 sorts (memory)0 sorts (disk)6075760 rows processed SQL> SELECT CASE OWNER WHEN 'SYSTEM' THEN 'SYSTEM'2 WHEN 'SYS' THEN 'SYSTEM' 3 ELSE 'USER' END 4 FROM T; 6075760 rows selected. Elapsed: 00:00:07.22 Execution Plan----------------------------------------------------------Plan hash value: 1601196873 --------------------------------------------------------------------------| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |--------------------------------------------------------------------------| 0 | SELECT STATEMENT | | 4245K| 68M| 13828 (1)| 00:03:14 || 1 | TABLE ACCESS FULL| T | 4245K| 68M| 13828 (1)| 00:03:14 |-------------------------------------------------------------------------- Note------ dynamic sampling used for this statement Statistics----------------------------------------------------------0 recursive calls0 db block gets47551 consistent gets0 physical reads0 redo size46288578 bytes sent via SQL*Net to client67317 bytes received via SQL*Net from client6077 SQL*Net roundtrips to/from client0 sorts (memory)0 sorts (disk)6075760 rows processed SQL> SELECT CASE WHEN OWNER = 'SYSTEM' THEN 'SYSTEM'2 WHEN OWNER = 'SYS' THEN 'SYSTEM' 3 ELSE 'USER' END 4 FROM T; 6075760 rows selected. Elapsed: 00:00:07.23 Execution Plan----------------------------------------------------------Plan hash value: 1601196873 --------------------------------------------------------------------------| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |--------------------------------------------------------------------------| 0 | SELECT STATEMENT | | 4245K| 68M| 13828 (1)| 00:03:14 || 1 | TABLE ACCESS FULL| T | 4245K| 68M| 13828 (1)| 00:03:14 |-------------------------------------------------------------------------- Note------ dynamic sampling used for this statement Statistics----------------------------------------------------------0 recursive calls0 db block gets47551 consistent gets0 physical reads0 redo size46288585 bytes sent via SQL*Net to client67317 bytes received via SQL*Net from client6077 SQL*Net roundtrips to/from client0 sorts (memory)0 sorts (disk)6075760 rows processed |
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