'select * from [table_name]'是一个游标吗？

Question

A few days back I was asked during an interview if select * from [table_name] is a cursor since it fetches a number of records at a time?

If yes then of which type? Explicit or Implicit and why ?

Please, if anyone can elaborate on this it would be a great help.

Answer 1

Oracle internally creates a cursor to serve your select query hence we call it IMPLICIT cursor.

Whereas we call the following an explicit cursor which you create on your own to have more controls on it.

CURSOR cur IS SELECT col FROM table WHERE condition;

Answer 2

The explicit cursors are those whcih explicitly are opened, fetched data, closed.

As stated, the usage of cursor have 2 parts:

1. definition of the cursor
2. usage of cursor

Definition of the cursor can be as

CURSOR c IS SELECT col1, col2 FROM table_name

or

FOR i IN (SELECT col1, col2 FROM table_name)

The differences comes next:

1. The explicit cursor needs

OPEN, FETCH, EXIT WHEN, CLOSE

1. The implicit cursor performs all above steps in the FOR LOOP statement.

Concluding, a SELECT statement is an implicit cursor.

Best, Mikcutu.

Answer 3

Yes, every query represents a cursor.

The Concepts Guide links from cursor to private SQL area , which it defines as

An area in memory that holds a parsed statement and other information for processing. The private SQL area contains data such as bind variable values, query execution state information, and query execution work areas.

The PL/SQL Language Reference says:

An implicit cursor is a session cursor that is constructed and managed by PL/SQL. PL/SQL opens an implicit cursor every time you run a SELECT or DML statement.

and

An explicit cursor is a session cursor that you construct and manage. You must declare and define an explicit cursor, giving it a name and associating it with a query (typically, the query returns multiple rows).

These are really concepts from PL/SQL, so I am not sure it is meaningful to use them for cursors from other clients such as a SQL*Plus or SQL Developer command line or a Java program. I'd say a cursor is only "explicit" if you name it in some PL/SQL code using the cursor keyword, and since you didn't (and can't) do that for a query from your command line, it's closer to the PL/SQL concept of an implicit cursor.