Declaring & Setting Variables in a Select Statement

Question

I'm attempting to write a simple query where I declare some variables and then use them in a select statement in Oracle. I've been able to do this before in SQL Server with the following:

DECLARE @date1   DATETIME
SET @date1 = '03-AUG-2010'

SELECT U.VisualID
FROM Usage u WITH(NOLOCK)
WHERE U.UseTime > @Date1

From the searching I've done it appears you can not declare and set variables like this in Select statements. Is this right or am I mssing something?

Answer 1

Try the to_date function.

Answer 2

The SET command is TSQL specific - here's the PLSQL equivalent to what you posted:

v_date1 DATE := TO_DATE('03-AUG-2010', 'DD-MON-YYYY');

SELECT u.visualid
  FROM USAGE u 
 WHERE u.usetime > v_date1;

There's also no need for prefixing variables with "@"; I tend to prefix variables with "v_" to distinguish between variables & columns/etc.

See this thread about the Oracle equivalent of NOLOCK...

Answer 3

From the searching I've done it appears you can not declare and set variables like this in Select statements. Is this right or am I missing something?

Within Oracle PL/SQL and SQL are two separate languages with two separate engines. You can embed SQL DML within PL/SQL, and that will get you variables. Such as the following anonymous PL/SQL block. Note the / at the end is not part of PL/SQL, but tells SQL*Plus to send the preceding block.

declare 
    v_Date1 date := to_date('03-AUG-2010', 'DD-Mon-YYYY');
    v_Count number;
begin
    select count(*) into v_Count
    from Usage
    where UseTime > v_Date1;
    dbms_output.put_line(v_Count);
end;
/

The problem is that a block that is equivalent to your T-SQL code will not work:

SQL> declare 
  2      v_Date1 date := to_date('03-AUG-2010', 'DD-Mon-YYYY');
  3  begin
  4      select VisualId
  5      from Usage
  6      where UseTime > v_Date1;
  7  end;
  8  /
    select VisualId
    *
ERROR at line 4:
ORA-06550: line 4, column 5:
PLS-00428: an INTO clause is expected in this SELECT statement

To pass the results of a query out of an PL/SQL, either an anonymous block or a stored procedure, cursors must be declare and the cursor is returned to the calling program. (Beyond the scope of answering this question. EDIT: see http://stackoverflow.com/questions/1170548/get-resultset-from-oracle-stored-procedure)

The client tool that connects to the database may have it's own bind variables. In SQL*Plus:

SQL> -- SQL*Plus does not all date type in this context
SQL> -- So using varchar2 to hold text
SQL> variable v_Date1 varchar2(20)
SQL>
SQL> -- use PL/SQL to set the value of the bind variable
SQL> exec :v_Date1 := '02-Aug-2010';

PL/SQL procedure successfully completed.

SQL> -- Converting to a date, since the variable is not yet a date.
SQL> -- Note the use of colon, this tells SQL*Plus that v_Date1
SQL> -- is a bind variable.
SQL> select VisualId
  2  from Usage
  3  where UseTime > to_char(:v_Date1, 'DD-Mon-YYYY');

no rows selected

Note the above is in SQL*Plus, may not (probably won't) work in Toad PL/SQL developer, etc. The lines starting with variable and exec are SQL*Plus commands. They are not SQL or PL/SQL commands. No rows selected because the table is empty.