PL-SQL Aptitude/ Interview Questions and Answers

1. Describe the difference between a procedure, function and anonymous pl/sql block.
   Level: Low
   Expected answer : Candidate should mention use of DECLARE statement, a function must return a value while a procedure doesn't have to.
   
   2. What is a mutating table error and how can you get around it?
   Level: Intermediate
   Expected answer: This happens with triggers. It occurs because the trigger is trying to update a row it is currently using. The usual fix involves either use of views or temporary tables so the database is selecting from one while updating the other.
   
   3. Describe the use of %ROWTYPE and %TYPE in PL/SQL
   Level: Low
   Expected answer: %ROWTYPE allows you to associate a variable with an entire table row.
   The %TYPE associates a variable with a single column type.
   
   4. What packages (if any) has Oracle provided for use by developers?
   Expected answer: Oracle provides the DBMS_ series of packages. There are many
   which developers should be aware of such as DBMS_SQL, DBMS_PIPE, DBMS_TRANSACTION,
   DBMS_LOCK, DBMS_ALERT, DBMS_OUTPUT, DBMS_JOB, DBMS_UTILITY, DBMS_DDL, UTL_FILE. If they can mention a few of these and describe how they used them, even better. If they include the SQL routines provided by Oracle, great, but not really what was asked.
   
   5. Describe the use of PL/SQL tables
   Expected answer: PL/SQL tables are scalar arrays that can be referenced by a
   binary integer. They can be used to hold values for use in later queries
   or calculations. In Oracle 8 they will be able to be of the %ROWTYPE designation, or RECORD.
   
   6. When is a declare statement needed ?
   The DECLARE statement is used in PL/SQL anonymous blocks such as with stand alone, non-stored PL/SQL procedures. It must come first in a PL/SQL stand alone file if it is used.
   
   7. In what order should a open/fetch/loop set of commands in a PL/SQL block be implemented if you use the NOTFOUND cursor variable in the exit when statement? Why?
   Expected answer: OPEN then FETCH then LOOP followed by the exit when. If not specified in this order will result in the final return being done twice because of the way the %NOTFOUND is handled by PL/SQL.
   
   8. What are SQLCODE and SQLERRM and why are they important for PL/SQL developers?
   Expected answer: SQLCODE returns the value of the error number for the last error encountered. The SQLERRM returns the actual error message for the last error encountered. They can be used in exception handling to report, or, store in an error log table, the error that occurred in the code. These are especially useful for the WHEN OTHERS exception.
   
   9. How can you find within a PL/SQL block, if a cursor is open?
   Expected answer: Use the %ISOPEN cursor status variable.
   
   10. How can you generate debugging output from PL/SQL?
   Expected answer: Use the DBMS_OUTPUT package. Another possible method is to just use the SHOW ERROR command, but this only shows errors. The DBMS_OUTPUT package can be used to show intermediate results from loops and the status of variables as the procedure is executed. The new package UTL_FILE can
   also be used.
   
   11. What are the types of triggers?
   Expected Answer: There are 12 types of triggers in PL/SQL that consist of
   combinations of the BEFORE, AFTER, ROW, TABLE, INSERT, UPDATE, DELETE and
   ALL key words:
   BEFORE ALL ROW INSERT
   AFTER ALL ROW INSERT
   BEFORE INSERT
   AFTER INSERT etc.