PL/SQL cursors vs. SQL*plus Select statement

Question

Hi folks, hope you're doing well,
Here is a question that kept me wondering:
Why would I use cursors when i can achieve the same thing with a SQL+ Select statement which is much easier to formulate than a cursor (e.g. you need no declaration, loops etc)?.
Thanks so much,
-a

Answer 1

There is no such thing as a SQL*Plus SELECT statement. The SELECT command is part of the SQL Language - not part of the SQL*Plus (very limited small vocabulary) macro language.
All SQL SELECTs (from client languages) winds up in the SQL Engine as SQL cursors. A SQL Cursor is basically the:
- SQL source code
- SQL "compiled" code (instructions on how to fetch the rows)
On the client side, client cursors (not to be confused with SQL cursors) are used. A client cursor is created in the client language when it makes SQL calls via the database client driver (called the OCI/Oracle Call Interface for Oracle clients).
Typically this is what a client does. It makes a connection to the database and gets a database handler in return. The database handler is the "communication channel" from the client to the db. In Oracle, the database handler in the client refers to the Oracle session (for that client) on db server.
A SQL statement (source code) is used by the client. This can be a SELECT statement you type in at the SQL*Plus command line. It can be a SELECT statement for the PL/SQL cursor command in a stored procedure.
The client creates a SQL handle for this SQL statement, by calling the Oracle client driver. Note that this SQL handle is a client handle - a client cursor for that SQL statement.
E.g.
a) sqlHandle = CreateSQL( databaseHandle, 'SELECT ... FROM ...')
b) sqlHandle.Parse
c) sqlHandle.Execute
After the SQL handle (client cursor) has been executed, the client can fetch rows from it.
This is what SQL*Plus does automatically for you, without you having to write the code to do it. SQL*Plus CONNECT command create a database connection handle. You enter a SELECT statement and SQL*Plus creates a SQL handle (client cursor), executes it, fetches from it, displays the rows, and closes the SQL handle when done.
The same applies to PL/SQL. You can use a SELECT statement just like that in PL/SQL. E.g.
declare
  i integer;
begin
  select count(*) into i from emp where deptid = 123;
end;This is called an implicit cursor. PL/SQL creates (just like SQL*Plus) an implicit client cursor. It creates and disposes of that client SQL handle for you - you do not need to do it.
Or, you can create an explicit cursor. E.g. declare
  cursor c is select count(*) from emp where deptid = 123;
  i integer;
begin
  open c;
  fetch c into i;
  close c;
end;The question as to when to use implicit (client) cursors versus explicit (client) cursors depends on your requirements. Do you need to cycle through the results of the SQL? Etc.
And keep in mind that in either case, the SQL Engine creates a SQL cursor anyway on its side.