plsql强化case/when/then

Oracle/PLSQL: Case Statement

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In Oracle 9i, you can use the case statement within an SQL statement. It has the functionality of an IF-THEN-ELSE statement.
译：在Oracle 9i中，你可以在SQL语句中使用case条件。它具有IF-THEN-ELSE条件的功能。
The syntax for the case statement is:
译：语法如下
CASE expression
WHEN condition_1 THEN result_1
WHEN condition_2 THEN result_2
...
WHEN condition_n THEN result_n
ELSE result END
expression is the value that you are comparing to the list of conditions. (ie: condition_1, condition_2, ... condition_n)
译：expression就是要与条件比较的值。（例如：条件condition_1, condition_2, ... condition_n）
condition_1 to condition_n must all be the same datatype. Conditions are evaluated in the order listed. Once a condition is found to be true, the case statement will return the result and not evaluate the conditions any further.
译：条件condition_1到 condition_n必须具有相同的数据类型。条件是按列表顺序赋值，一旦一个条件为真了，case条件就会返回结果并且不会再次去执行该条件了
result_1 to result_n must all be the same datatype. This is the value returned once a condition is found to be true.
译：结果result_1到 result_必须具有相同的数据类型。一旦有条件成立，这就是返回的值。
Note:
注：
If no condition is found to be true, then the case statement will return the value in the ELSE clause.
译：如果没有找到为真的条件，case将返回ELSE条件中值。
If the ELSE clause is omitted and no condition is found to be true, then the case statement will return NULL.
译：如果省略了ELSE条件并且也没有找到为真的条件，case条件将返回NULL。
You can have up to 255 comparisons in a case statement. Each WHEN ... THEN clause is considered 2 comparisons.
译：在case条件中最多可以有255个比较，每个WHEN ... THEN子被认为两次比较。
For Example:
You could use the case statement in an SQL statement as follows:
译：你可以在SQL语句中像如下使用case语句：
select table_name,
CASE owner
WHEN 'SYS' THEN 'The owner is SYS'
WHEN 'SYSTEM' THEN 'The owner is SYSTEM'
ELSE 'The owner is another value' END
from all_tables;
 
The above case statement is equivalent to the following IF-THEN-ELSE statement:
译：下面的case条件等价于下面的IF-THEN-ELSE语句：
IF owner = 'SYS' THEN
     result := 'The owner is SYS';
ELSIF owner = 'SYSTEM' THEN
    result := 'The owner is SYSTEM'';
ELSE
    result := 'The owner is another value';
END IF;
 
The case statement will compare each owner value, one by one.
译：case条件将一个一个的比较每个所有者的值：
One thing to note is that the ELSE clause within the case statement is optional. You could have omitted it. Let's take a look at the SQL statement above with the ELSE clause omitted.
译：要注意的一个就是ELSE在case条件中可选的，你可以省略它。让我们看一个关于上面语句省略ELSE的SQL语句。
Your SQL statement would look as follows:
select table_name,
CASE owner
WHEN 'SYS' THEN 'The owner is SYS'
WHEN 'SYSTEM' THEN 'The owner is SYSTEM' END
from all_tables;
With the ELSE clause omitted, if no condition was found to be true, the case statement would return NULL.
译：省略了ELSE条件，且没有找到为真的条件，case就会返加NULL