What is the difference among "dropping a table", "truncating a table" and "deleting all records" from a table.

Dropping : (Table structure + Data are deleted), Invalidates the dependent objects ,Drops the indexes
Truncating: (Data alone deleted), Performs an automatic commit, Faster than delete

Delete : (Data alone deleted), Doesn’t perform automatic commit

Difference between a "where" clause and a "having" clause.

Having clause is used only with group functions whereas Where is not used with.

What's the difference between a primary key and a unique key?

Both primary key and unique enforce uniqueness of the column on which they are defined. But by default primary key creates a clustered index on the column, where are unique creates a nonclustered index by default. Another major difference is that, primary key doesn't allow NULLs, but unique key allows one NULL only.

What are cursors? Explain different types of cursors. What are the disadvantages of cursors? How can you avoid cursors?

Cursors allow row-by-row prcessing of the resultsets.

Types of cursors: Static, Dynamic, Forward-only, Keyset-driven. See books online for more information.

Disadvantages of cursors: Each time you fetch a row from the cursor, it results in a network roundtrip, where as a normal SELECT query makes only one rowundtrip, however large the resultset is. Cursors are also costly because they require more resources and temporary storage (results in more IO operations). Furthere, there are restrictions on the SELECT statements that can be used with some types of cursors.

Most of the times, set based operations can be used instead of cursors.

What are triggers? How to invoke a trigger on demand?

Triggers are special kind of stored procedures that get executed automatically when an INSERT, UPDATE or DELETE operation takes place on a table.

Triggers can't be invoked on demand. They get triggered only when an associated action (INSERT, UPDATE, DELETE) happens on the table on which they are defined.

Triggers are generally used to implement business rules, auditing. Triggers can also be used to extend the referential integrity checks, but wherever possible, use constraints for this purpose, instead of triggers, as constraints are much faster.

What is a join and explain different types of joins.

Joins are used in queries to explain how different tables are related. Joins also let you select data from a table depending upon data from another table.

Types of joins: INNER JOINs, OUTER JOINs, CROSS JOINs. OUTER JOINs are further classified as LEFT OUTER JOINS, RIGHT OUTER JOINS and FULL OUTER JOINS.

Different types of table join

There are 3 types of joins basis on Oracle 8i



Simple join

1. Equi join : Equi join is one which contains comparision operator '=' in where clause.

2. Non Equi join: If the comparision operator in the join condition is other than '=' then it is ....


Self join : Joining a table to itself can be done by..



Outer join : To retrieve rows which do not satisfy a query, the outer join concept can be used.

How do you handle exceptions and errors in stored procedures?

exceptions can be handled in various ways in the stored procedures. Method (1) could be using EXCEPTION block in the procedure within which use WHEN THEN. Method (2) could be declare a variable in DECLARE section as EXCEPTION type, raise it wherever required using RAISE and handle it in EXCEPTION part using WHEN clause. Method (3) could be using the keywords SQLCODE & SQLERRM to display error no and error code..

What rule optimizations are possible in SQL query

For optimization of a sql query, u first need to check the bottleneck and then taste the query with CBO/RBO. Using the option which is best for ur environment. For CBO to work , u need to analyze the table first then only it will have some effect on the performance of the query.