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What is Normalization?
Database normalization is a data design and organization process applied to data structures based on rules that help building rlational databases. In relational database design, the process of organizing data to minimize redundancy is called normalization. Normalization usually involves dividing a database into two or more tables and defining relationships between the tables. The objective is to isolate data so that additions, deletions, and modifications of a field can be made in just one table and then propagated through the rest of the database via the define relationships.
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What is De‐normalization?
De‐normalization is the process of attempting to optimize the performance of a database by adding redundant data. It is sometimes necessary because current DBMSs implement the relational model poorly. A true relational DBMS would allow for a fully normalized database at the logical level, while providing physical storage of data that is tuned for high performance. De‐normalization is a technique to move from higher to lower normal forms of database modeling in order to speed up database acces.
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What is Stored Procedure?
A stored procedure is a named group of SQL statements that have been previously created and stored in the server database. Stored procedures accept input parameters so that a single procedure can be used over the network by several clients using diferent input data. And when the procedure is modified, all clients automatically get the new version. Stored procedures reduce network traffic and improve performance. Stored procedures can be used to help ensure the integrity of the database. e.g. sp_helpdb, sp_renamedb, sp_depends etc
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What is Trigger?
A trigger is a SQL procedure that initiates an action when an event (INSERT, DELETE or UPDATE) occurs. Triggers are stored in and managed by the DBMS. Triggers are used to maintain the referential integrity of data by changing the data in a systematic fashion. A trigger cannot be called or executed; DBMS automatically fires the trigger as a result of a data modification to the associated table. Triggers can be viewed as similar to stored procedures in that both consist of procedural logic that is stored at the databaselevel. Stored procedures, however, are not event‐drive and are not attached to a specific table as triggers are. Stored procedures are explicitly executed by invoking a CALL to the procedure while triggers are implicitly executed. In addition, triggers can also execute stored procedures.
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What are different Types of Join?
Cross Join
A cross join that does not have a WHERE clause produces the Cartesian product of the tables involved in the join. The size of a Cartesian product result set is the number of rows in the first table multiplied by the number of rows in the seond table. The common example is when company wants to combine each product with a pricing table to analyze each product at each price.
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Inner Join
A join that displays only the rows that have a match in both joined tables is known as inner Join. This is the default type of join in the Query and View Designer. Outer Join A join that includes rows even if they do not have related rows in the joined table is an Outer Join. You can create three different outer join to specify the unmatched rows to be included: • Left Outer Join: In Left Outer Join all rows in the first‐named table i.e. "left" table, which appears leftmost in the JOIN clause are included. Unmatched rows in the right table do not appear.
• Right Outer Join: In Right Outer Join all rows in the second‐named table i.e. "right" table, which appears rightmost in the JOIN clause are included. Unmatched rows in the left table are not included.
• Full Outer Join:
In Full Outer Join all rows in all joined tables are included, whether they are matched or not.
Self Join This is a particular case when one table joins to itself, with one or two aliases to avoid confusion. A self join can be of any type, as long as the joined tables are the same. A self join is rather unique in that it involves a relationship with only one table. The common example is when company has a hierarchal reporting structure whereby one member of staff reports to another.
Self Join can be Outer Join or Inner Join. (Read More Here)
What are primary keys and foreign keys?
Primary keys are the unique identifiers for each row. They must contain unique values and cannot be null. Due to their importance in relational databases, Primary keys are the most fundamental of all keys and constraints. A table can have only one Primary key.
Foreign keys are both a method of ensuring data integrity and a manifestation of the relationship between tables.
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What is UNIQUE KEY constraint?
A UNIQUE constraint enforces the uniqueness of the values in a set of columns, so no duplicate values are entered. The unique key constraints are used to enforce entity integrity as the primary key constraints.
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Q-Retrieve the duplicate rows from the database
SELECT name,
COUNT(name) AS NumOccurrences
FROM test
GROUP BY name
HAVING ( COUNT(name) > 1 )
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--second max salary
SELECT * FROM Employee E1 WHERE 1 =
(SELECT COUNT(DISTINCT Salary) FROM Employee E2 WHERE E1.Salary < E2.Salary)
--second lowest salary
SELECT * FROM Employee E1 WHERE 1 =
(SELECT COUNT(DISTINCT Salary) FROM Employee E2 WHERE E1.Salary > E2.Salary)
--To find nth lowest salary
SELECT * FROM Employee E1 WHERE n-1 =
(SELECT COUNT(DISTINCT Salary) FROM Employee E2 WHERE E1.Salary > E2.Salary)
