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

Language Integrated Query (LINQ) adds the ability to query objects using .NET languages. The LINQ to SQL object/relational mapping (O/RM) framework provides the following basic features:

- Tools to create classes (usually called entities) mapped to database tables

- Compatibility with LINQ's standard query operations

- The DataContext class, with features such as entity record monitoring, automatic SQL statement generation, record concurrency detection, and much more

They get stored in System Catalog Views sys.server_principals and sys.sql_logins.

Properties of Sub-Query

- A sub-query must be enclosed in the parenthesis.

- A sub-query must be put in the right hand of the comparison operator, and

- A sub-query cannot contain an ORDER-BY clause.

- A query can contain more than one sub-query.

Types of Sub-Query

- Single-row sub-query, where the sub-query returns only one row.

- Multiple-row sub-query, where the sub-query returns multiple rows,. and

- Multiple column sub-query, where the sub-query returns multiple columns

Log shipping is the process of automating the backup of database and transaction log files on a production SQL server, and then restoring them onto a standby server. Enterprise Editions only supports log shipping. In log shipping the transactional log file from one server is automatically updated into the backup database on the other server. If one server fails, the other server will have the same db and can be used this as the Disaster Recovery plan. The key feature of log shipping is that it will automatically backup transaction logs throughout the day and automatically restore them on the standby server at defined interval.

SQL Server can be linked to any server provided it has OLE-DB provider from Microsoft to allow a link. E.g. Oracle has an OLE-DB provider for oracle that Microsoft provides to add it as linked server to SQL Server group.

Delete command removes the rows from a table based on the condition that we provide with a WHERE clause. Truncate will actually remove all the rows from a table and there will be no data in the table after we run the truncate command.

TRUNCATE:

- TRUNCATE is faster and uses fewer system and transaction log resources than DELETE.

- TRUNCATE removes the data by deallocating the data pages used to store the table's data, and only the page deallocations are recorded in the transaction log.

- TRUNCATE removes all rows from a table, but the table structure, its columns, constraints, indexes and so on, remains. The counter used by an identity for new rows is reset to the seed for the column.

- You cannot use TRUNCATE TABLE on a table referenced by a FOREIGN KEY constraint. Because TRUNCATE TABLE is not logged, it cannot activate a trigger.

- TRUNCATE cannot be rolled back.

- TRUNCATE is DDL Command.

- TRUNCATE Resets identity of the table

DELETE:

- DELETE removes rows one at a time and records an entry in the transaction log for each deleted row.

- If you want to retain the identity counter, use DELETE instead. If you want to remove table definition and its data, use the DROP TABLE statement.

- DELETE Can be used with or without a WHERE clause

- DELETE Activates Triggers.

- DELETE can be rolled back.

- DELETE is DML Command.

- DELETE does not reset identity of the table.

Note: DELETE and TRUNCATE both can be rolled back when surrounded by TRANSACTION if the current session is not closed. If TRUNCATE is written in Query Editor surrounded by TRANSACTION and if session is closed, it can not be rolled back but DELETE can be rolled back.

- 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.