All Sql Interview Questions

2 What is Cursor? How to use a Cursor?

A database cursor is a control structure that allows for the traversal of records in a database. Cursors, in addition, facilitates processing after traversal, such as retrieval, addition, and deletion of database records. They can be viewed as a pointer to one row in a set of rows.

Working with SQL Cursor:

  • DECLARE a cursor after any variable declaration. The cursor declaration must always be associated with a SELECT Statement.
  • Open cursor to initialize the result set. The OPEN statement must be called before fetching rows from the result set.
  • FETCH statement to retrieve and move to the next row in the result set.
  • Call the CLOSE statement to deactivate the cursor.
  • Finally use the DEALLOCATE statement to delete the cursor definition and release the associated resources.
  • What can you tell about WAL (Write Ahead Logging)?

    Write Ahead Logging is a feature that increases the database reliability by logging changes before any changes are done to the database. This ensures that we have enough information when a database crash occurs by helping to pinpoint to what point the work has been complete and gives a starting point from the point where it was discontinued.

    For more information, you can refer here.

    How to delete a column in SQL?

    To delete a column in SQL we will be using DROP COLUMN method:

    We will start off by giving the keywords ALTER TABLE, then we will give the name of the table, following which we will give the keywords DROP COLUMN and finally give the name of the column which we would want to remove.

    Explain Equi join with an example.

    When two or more tables have been joined using equal to operator then this category is called an equi join. Just we need to concentrate on the condition is equal to (=) between the columns in the table.


    1 What is an Index? Explain its different types.

    A database index is a data structure that provides a quick lookup of data in a column or columns of a table. It enhances the speed of operations accessing data from a database table at the cost of additional writes and memory to maintain the index data structure.

    There are different types of indexes that can be created for different purposes:

    Unique indexes are indexes that help maintain data integrity by ensuring that no two rows of data in a table have identical key values. Once a unique index has been defined for a table, uniqueness is enforced whenever keys are added or changed within the index.

    Non-unique indexes, on the other hand, are not used to enforce constraints on the tables with which they are associated. Instead, non-unique indexes are used solely to improve query performance by maintaining a sorted order of data values that are used frequently.

    Clustered indexes are indexes whose order of the rows in the database corresponds to the order of the rows in the index. This is why only one clustered index can exist in a given table, whereas, multiple non-clustered indexes can exist in the table.

    The only difference between clustered and non-clustered indexes is that the database manager attempts to keep the data in the database in the same order as the corresponding keys appear in the clustered index.

    Clustering indexes can improve the performance of most query operations because they provide a linear-access path to data stored in the database.

    Top 25 SQL Interview Questions and Answers(The BEST SQL Interview Questions)

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