Placement Papers: CMC Oracle Test

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What are the difference b/w candidate key, primary key and unique key?
What is difference b/w pre query and post query?
How many number of columns can be created in a single table.
What is meant by ROWID? Why we need it?
What is transaction?
Difference b/w function, procedure?
Which one is the best way to find out the number of rows in a table, state by following count 1 and count ( * ), count (rowed)
One table is given and questions based on this table.
Write query to delete a single column in a table.
Write query to add one more column in a existing table.
Write query to delete only 2 duplicate records in a table. But the table they have given contains 3 duplicate records. How to do it.
Some SELECT statements queries they asked like using group by, ordered by, where etc.