Oracle/PLSQL: Case Statement

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