Oracle的CASE表示式

現在Oracle支援兩種CASE表示方式，一種Oracle稱為simple CASE（簡單形式）,另外一種為searched CASE（查詢形式）。simple CASE相當於使用函式Decode，但是Case一般用於操作更為複雜的語句。CASE的形式更易於閱讀。

simple CASE的表示形式為：

CASE expr WHEN comparison_expr THEN return_expr
[, WHEN comparison_expr THEN return_expr]... [ELSE else_expr] END

*注：[]裡面的內容可以省略

執行方式為：

Oracle檢查表示式expr是否和表示式comparison_expr相等，如果相等則執行return_expr，否則執行ELSE else_expr的內容。

searched CASE表示形式為：

CASE WHEN condition THEN return_expr [, WHEN condition THEN return_expr]
   ... ELSE else_expr] END

 可以使用任何一種方式。

這兩種表示方式都是最多支援255個引數，其中每對When...Then算作2個引數。

例如：

1、

SELECT AVG(CASE when e.salary > 2000 THEN e.salary ELSE 2000 end)
  AS avg_sal_2k_floor
FROM employees e;
2、

SELECT AVG(CASE WHEN e.sal BETWEEN 0 AND 2000 THEN e.sal ELSE null END) avg2000,
    AVG(CASE WHEN e.sal BETWEEN 2001 AND 5000 THEN e.sal ELSE null END) avg5000
FROM emps e;

3、

SELECT SUM(CASE WHEN cust_credit_limit BETWEEN  0 AND 3999 THEN 1 ELSE 0 END)
  AS "0-3999",
SUM(CASE WHEN cust_credit_limit BETWEEN  4000 AND 7999 THEN 1 ELSE 0 END)
  AS "4000-7999",
SUM(CASE WHEN cust_credit_limit BETWEEN  8000 AND 11999 THEN 1 ELSE 0 END)
  AS "8000-11999",
SUM(CASE WHEN cust_credit_limit BETWEEN  12000 AND 16000 THEN 1 ELSE 0 END)
  AS "12000-16000"
FROM customers WHERE cust_city = 'Marshal';

    0-3999  4000-7999 8000-11999 12000-16000
---------- ---------- ---------- -----------
         8          7          7           1

4、

SELECT (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