SQL Questions

The knowledge of Joins is a MUST for every interviewee. Most SQL programmers have used inner join and  outer join[left/right]; but the catch point here is to also mention cross join and self-join.

SELECT DISTINCT(salary) FROM emp as e1 

WHERE (3) = (SELECT COUNT(DISTINCT(salary)) FROM emp as e2  WHERE e1.salary <= e2.salary)

UNION selects only distinct values whereas UNION ALL selects all values and not just distinct ones

An SQL statement is a string of characters that conforms to the syntax and format specified in the international standard.

A query is a statement that returns a record-set.

One of the most common errors that you might face in join statements is 

Server: Msg 4104, Level 16, State 1, Line 1
The multi-part identifier could not be bound.

A multipart identifier is any description of a field or table that contains multiple parts - for instance MyTable.SomeRow - if it can't be bound that means there's something wrong with it - either you've got a simple typo, or a confusion between table and column. It can also be caused by using reserved words in your table or field names and not surrounding them with [].

The main reason for this error is that the source table cannot be found, for example if you have statement such as Table1.OrderDate, and then if you get error above, this means that Table1 cannot be found in the query. Sometimes you can see that source table exists in the query, but T-SQL cannot understand it, especially when you write join statements.

- A clustered index is a special type of index that reorders the way records in the table are physically stored. Therefore table can have only one clustered index. The leaf nodes of a clustered index contain the data pages.

- A non clustered index is a special type of index in which the logical order of the index does not match the physical stored order of the rows on disk. The leaf node of a non clustered index does not consist of the data pages. Instead, the leaf nodes contain index rows.

Aggregate functions perform a calculation on a set of values and return a single value. Aggregate functions ignore NULL values except COUNT function. HAVING clause is used, along with GROUP BY, for filtering query using aggregate values.

Following functions are aggregate functions. 

AVG, MIN CHECKSUM_AGG, SUM, COUNT, STDEV, COUNT_BIG, STDEVP, GROUPING, VAR, MAX. VARP

Cursor_Already_Open,  Invalid_Cursor.