SQL Interview Questions – Part 4

What is a Cartesian product? What causes it?

A Cartesian product is the result of an unrestricted join of two or more tables. The result set of a three table Cartesian product will have x * y * z number of rows where x, y, z correspond to the number of rows in each table involved in the join. It is causes by specifying a table in the FROM clause without joining it to another table.

What is an advantage to using a stored procedure as opposed to passing an SQL query from an application.

A stored procedure is pre-loaded in memory for faster execution. It allows the DBMS control of permissions for security purposes. It also eliminates the need to recompile components when minor changes occur to the database.

What is the difference of a LEFT JOIN and an INNER JOIN statement?

A LEFT JOIN will take ALL values from the first declared table and matching values from the second declared table based on the column the join has been declared on. An INNER JOIN will take only matching values from both tables

When a query is sent to the database and an index is not being used, what type of execution is taking place?

A table scan.

What are the pros and cons of using triggers?

A trigger is one or more statements of SQL that are being executed in event of data modification in a table to which the trigger belongs.
Triggers enhance the security, efficiency, and standardization of databases.
Triggers can be beneficial when used:
— to check or modify values before they are actually updated or inserted in the database. This is useful if you need to transform data from the way the user sees it to some internal database format.
— to run other non-database operations coded in user-defined functions
— to update data in other tables. This is useful for maintaining relationships between data or in keeping audit trail information.
— to check against other data in the table or in other tables. This is useful to ensure data integrity when referential integrity constraints aren’t appropriate, or when table check constraints limit checking to the current table only.



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