What is the use of COALESCE in SQL Server

Let us understand the use of COALESCE with the help of an example.

In this example, the Candidate table is shown to include three columns with information about a Candidate: 
1. Candidate_id
2. PrimaryEmail
3. SecondaryEmail

COALESCE in the SELECT statement below, selects the PrimaryEmail if it is not null. If the PrimaryEmail is null then SecondaryEmail will be selected. If both PrimaryEmail and SecondaryEmail is present then only PrimaryEmail is selected. So, COALESCE returns the first nonnull column among the list of columns passed. If both PrimaryEmail and SecondaryEmail is NULL, COALESCE returns NULL.



COALESCE can also be used in joins as shown in the example below. If the Candidate table has a non null value in the Email column, then the value is selected. If the Email column is null in the Candidate Table then, CompanyEmail from CandidateCompany Table is selected.

What is the difference between a Temporary Table and a Table Variable

What is the difference between a Temporary Table and a Table Variable?
Or
When do you use Table Variable over a Temporary Table and vice versa?

1. Table variable is created in the memory where as a temporary table is created in the TempDB. But, if there is a memory pressure, the pages belonging to a table variable may be pushed out to tempdb.

2. Table variables cannot be involved in transactions, logging or locking. This makes table variable faster than a temporary table.

3. You can pass table variable as parameter to functions and stored procedures, where as you cannot do the same with temporary table.

4. A temporary table can have indexes, whereas a table variable can only have a primary index. If speed is an issue Table variables can be faster, but if there are a lot of records, or there is a need to search the temporary table based on a clustered index, then a Temporary Table would be better. If you have less than 100 rows generally use a table variable. Otherwise use a temporary table. This is because SQL Server won't create statistics on table variables.

Read this article for more SQL Server Interview Questions on Temporary Tables.

SQL Server Interview Questions on Indexes - Part 2

What are the disadvantages of an Index?
There are 2 disadvantages of an Index
1. Increased Disk Space
2. Insert, Update and Delete statements could be slow. In short, all DML statements could be slow.

Disk Space: Indexes are stored on the disk, and the amount of space required will depend on the size of the table, and the number and types of columns used in the index. Disk space is generally cheap enough to trade for application performance, particularly when a database serves a large number of users. 

Insert, Update and Delete statements could be slow: Another downside to using an index is the performance implication on data modification statements. Any time a query modifies the data in a table (INSERT, UPDATE, or DELETE), the database needs to update all of the indexes where data has changed. Indexing can help the database during data modification statements by allowing the database to quickly locate the records to modify, however, providing too many indexes to update can actually hurt the performance of data modifications. This leads to a delicate balancing act when tuning the database for performance.

What are the 2 types of Indexes in SQL Server?
1. Clustered Index 
2. Non Clustered Index

How many Clustered and Non Clustered Indexes can you have per table?
Clustered Index - Only one Clustered Index per table. A clustered index contains all of the data for a table in the index, sorted by the index key. Phone Book is an example for Clustered Index.
Non Clustered Index - You can have multiple Non Clustered Indexes per table. Index at the back of a book is an example for Non Clustered Index.

Which Index is faster, Clustered or Non Clustered Index?
Clustered Index is slightly faster than Non Clustered Index. This is because, when a Non Clustered Index is used there is an extra look up from the Non Clustered Index to the table, to fetch the actual rows.



When is it usually better to create a unique nonclustered index on the primary key column?
Sometimes it is better to use a unique nonclustered index on the primary key column, and place the clustered index on a column used by more queries. For example, if the majority of searches are for the price of a product instead of the primary key of a product, the clustered index could be more effective if used on the price field.

What is a Composite Index in SQL Server?
or 
What is the advantage of using a Composite Index in SQL Server?
or 
What is Covering Query?
A composite index is an index on two or more columns. Both clustered and nonclustered indexes can be composite indexes.
If all of the information for a query can be retrieved from an Index. A clustered index, if selected for use by the query optimizer, always covers a query, since it contains all of the data in a table.

By creating a composite indexes, we can have covering queries.

SQL Server Interview Questions on Indexes - Part 1

What is the use of an Index in SQL Server?
Relational databases like SQL Server use indexes to find data quickly when a query is processed. Creating the proper index can drastically increase the performance of an application.
What is a table scan?
or 
What is the impact of table scan on performance?
When a SQL Server has no index to use for searching, the result is similar to the reader who looks at every page in a book to find a word. The SQL engine needs to visit every row in a table. In database terminology we call this behavior a table scan, or just scan. A full table scan of a very large table can adversely affect the performance. Creating proper indexes will allow the database to quickly narrow in on the rows to satisfy the query, and avoid scanning every row in the table. 

What is the system stored procedure that can be used to list all the indexes that are created for a specific table?
sp_helpindex is the system stored procedure that can be used to list all the indexes that are created for a specific table.

For example, to list all the indexes on table tblCustomers, you can use the following command.
EXEC sp_helpindex tblCustomers

What is the purpose of query optimizer in SQL Server?
An important feature of SQL Server is a component known as the query optimizer. The query optimizer's job is to find the fastest and least resource intensive means of executing incoming queries. An important part of this job is selecting the best index or indexes to perform the task.

What is the first thing you will check for, if the query below is performing very slow?
SELECT * FROM tblProducts ORDER BY UnitPrice ASC

Check if there is an Index created on the UntiPrice column used in the ORDER BY clause. An index on the UnitPrice column can help the above query to find data very quickly.When we ask for a sorted data, the database will try to find an index and avoid sorting the results during execution of the query. We control sorting of a data by specifying a field, or fields, in an ORDER BY clause, with the sort order as ASC (ascending) or DESC (descending). 

With no index, the database will scan the tblProducts table and sort the rows to process the query. However, if there is an index, it can provide the database with a presorted list of prices. The database can simply scan the index from the first entry to the last entry and retrieve the rows in sorted order. 

The same index works equally well with the following query, simply by scanning the index in reverse.
SELECT * FROM tblProducts ORDER BY UnitPrice DESC 



What is the significance of an Index on the column used in the GROUP BY clause?
Creating an Index on the column, that is used in the GROUP BY clause, can greatly improve the perofrmance. We use a GROUP BY clause to group records and aggregate values, for example, counting the number of products with the same UnitPrice. To process a query with a GROUP BY clause, the database will often sort the results on the columns included in the GROUP BY. 

The following query counts the number of products at each price by grouping together records with the same UnitPrice value.
SELECT UnitPrice, Count(*) FROM tblProducts GROUP BY UnitPrice 

The database can use the index (Index on UNITPRICE column) to retrieve the prices in order. Since matching prices appear in consecutive index entries, the database is able to count the number of products at each price quickly. Indexing a field used in a GROUP BY clause can often speed up a query.

What is the role of an Index in maintaining a Unique column in table?
Columns requiring unique values (such as primary key columns) must have a unique index applied. There are several methods available to create a unique index. 
1. Marking a column as a primary key will automatically create a unique index on the column.
2. We can also create a unique index by checking the Create UNIQUE checkbox when creating the index graphically. 
3. We can also create a unique index using SQL with the following command: 
CREATE UNIQUE INDEX IDX_ProductName On Products (ProductName)

The above SQL command will not allow any duplicate values in the ProductName column, and an index is the best tool for the database to use to enforce this rule. Each time an application adds or modifies a row in the table, the database needs to search all existing records to ensure none of values in the new data duplicate existing values.