什么是scalar subquery

What are scalar subqueries?

Scalar subqueries are a great feature of Oracle, and is another example of how Oracle's SQL implementation really does set itself apart from all the rest. Scalar subqueries allow you to embed SQL statements which return a scalar value "within" SQL statements, otherwise known as "SELECTing a SELECT".

The simplest example is something like this :

SQL> SELECT
  2    ( SELECT sysdate FROM dual )
  3  FROM dual;

(SELECTSY
---------
07-MAR-06

So, as you can see, no longer do you need PL/SQL functions to issue a cursor and return a value, you can embed it directly in the SELECT statement.

So, what advantages does this bring? Well, there are many, from the simplifying of cursors, to the reduction of context switching . I go through the major reasons why you may want to consider this type of approach in the following sections.

 

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