How to perform a select op on a table whose name is contained by a variable

Question

I am using Oracle SQL*Plus client.
declare
tablename_var varchar(10); LOOK HERE
date_var date;
begin
select sysdate
into date_var
from &tablename_var; LOOK HERE
dbms_output.put_line('The system date is '||date_var);
end;
The above code prompts the user to enter a value for the variable tablename_var, once the user enters 'dual' on prompt... The system date is given as output.
declare
tablename_var varchar(10):='dual'; LOOK HERE
date_var date;
begin
select sysdate
into date_var
from tablename_var; LOOK HERE
dbms_output.put_line('The system date is '||date_var);
end;
This code fails and gives an error. I understand. Engine interprets it as... trying to perform a select operation on a variable, and says a table or a view is expected.
Can someone please tell me what to do, to perform a select operation on a table whose name is contained by a variable, like above?

Answer 1

Hi,
Welcome to the forum!
You need Dynamic SQL if the table- or column names are variables.
Getting the current date isn't a good example; you could get that without a query simply by saying
date_var := SYSDATE;Also, SELECT ... INTO will raise an error if the query does not return exactly 1 row.
In the example below, let's get the latest entry_date from a given table:
For example:
declare
     tablename_var     varchar(10);
     date_var      date;
     sql_txt           VARCHAR2 (1000);
begin
     tablename_var := 'table_x;
     sql_txt := 'select  MAX (entrydate)'
          || '  from  ' || tablename_var;
     dbms_output.put_line (sql_txt || ' = sql_txt');    -- Not essential, but recommended
     EXECUTE IMMEDIATE  sql_txt  INTO date_var;
     dbms_output.put_line ('The latest entry_date is ' || date_var);
end;Dynamic SQL involves creating a string that contains the SQL statement. Displaying the string is purely optional, of course, but it's a very good idea when writing and de-bugging code. Remember to comment-out or delete the display before moving the code into Production.
For an exercise, make the column name a variable, instead of the hard-coded entry_date.
Edited by: Frank Kulash on Nov 5, 2009 9:20 AM