SQL Interview Questions – Part-3

What is a primary key?
A primary key is a unique column (or set of columns) in a relational database table that is used to identify each record in the table.

Entity integrity makes sure that there aren’t any duplicate records in the database and that each record’s identifying field is distinct and never null. The primary tenet of entity integrity is the existence of the Primary Key. It cannot be a relational table if an entity key is not present. According to the relational model, each relation (or table) must have a primary key (abbreviated PK) that allows each row of the same relation to be identified by its content, that is, by a distinct and minimal value.

What is the CHECK constraint?
In SQL Server, you may create a check constraint in a table to specify the data values that are permitted in one or more columns. This means, that when you define a CHECK constraint on a column, it restricts the values that can be assigned to that column.

What is the difference between the Primary Key and Unique Key?

  • A primary key can be one or more table fields that uniquely identify each record in the table. Alternatively, a unique key prevents two rows from having identical values in a column.
  • In a relational database, a table can have several unique keys, but it is not possible for a table to have more than one primary key.
  • Unlike a unique key, which can have NULL values, a primary key column can only have one NULL value per table.
  • A primary key should be unique, but a unique key cannot necessarily be the primary key.
  • Data is physically arranged in a sequential index, with the primary key being a clustered index. The unique key, on the other hand, is a singular non-clustered index.
  • While the unique key enforces unique data, the primary key implements entity integrity.

What is the DEFAULT Constraint?
The DEFAULT constraint is used to provide a column’s default value. If no alternative value is supplied, that is, if the user leaves the column blank, the default value will be appended to all new records.

What is the FOREIGN KEY Constraint in SQL?
To establish a connection between two or more tables, the foreign key constraints will be used. One table’s primary key column can be referred to another table’s column as a foreign key. It is to enforce a constraint to match the foreign key column data-type and values with the parent key column data-type and values. The foreign key columns are used in joins while retrieving the data as the relationship between the tables indicates that the tables have been optimized to be combined.

Is there any limitation on having FOREIGN KEYs in a table?
There is no specific limitation to have number of foreign key constraints in a table, however, it is recommended to have not more than 253 foreign key constraints in a table. We can create a foreign key by defining REFERENCES or FOREIGN KEY constraints while creating or modifying a table.

What is indexing in SQL Server?
An index in SQL is a quick lookup table used to find records that users frequently search for. An index is designed to be small, fast, and optimized for quick lookups. It is extremely useful for linking relational tables and searching large tables.

In other words, an index is a collection of keys derived from one or more columns in a table or view. These keys are stored in a structure (B-tree) that allows SQL Server to quickly and efficiently find the row or rows associated with the key values.

How many different types of indexes does SQL Server support?
The indexes are classified into clustered, non-clustered, unique Index, spatial, column store, filtered, and hash indexes. However, widely used indexes are clustered, unique and non-clustered indexes.

What is clustered index?
A clustered index is an index that specifies the physical arrangement of a database’s table records. There can only be one clustered index per table since there can only be one method that records are physically stored in a database table. On a primary key column, a clustered index is automatically built. Only tables are sorted by clustered indexes. They don’t require more storage as a result.

What is a non-clustered index?
Non-clustered indexes take up more storage because they are kept apart from the real table. Since there is no additional lookup step required, clustered indexes are quicker than non-clustered ones.

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