Monthly Archives: June 2015


Generate serial number for each consecutive set of numbers 2

In this post, we are going to generate serial number for each consecutive set of numbers in a given column. We have a table tbl_Donation with columns EmpId and DonationYear. For each consecutive set of years of each employee, we need to generate serial numbers. If employee “X” donated in 2006, 2007 and 2008 years consecutively and then in 2010 and 2011, we have to assign 1 for 2005, 2 for 2006, 3 for 2007 followed by 1 for 2010 and 2 for 2011. We need to generate incremental series for each consecutive set of years but as and when a gap occurs between donation years, series has to be restarted with 1.

PARTITION BY clause in a ROW_NUMBER() window function could be able to generate the required output, if we had same number along with donation years for each consecutive set of years for all employees. Initially, we … More


Database normalization

In previous chapter, “Types of keys“, we had a good discussion on keys and their types in SQL Server. In this chapter “Database normalization”, we are going to discuss database normalization process and normal forms. Lets start our discussion with definition of Normalization.

Normalization

Normalization is a process of minimizing redundant data from database by decomposing the columns of a table into multiple tables. This process increases the number of tables in the database but it minimizes the redundant (duplicate), inaccurate and inconsistent data in database. Decomposed tables are connected using relationships (primary keys and foreign keys) to reduce the insert, update and delete anomalies. During normalization process, database designer decides the actual structure of the tables and their relationships. It is easy to find and fix any design problem at this early stage. During normalization process, we have various normal forms with some conditions. A table … More


CHAR, NCHAR, VARCHAR and NVARCHAR default length

What is the default length of CHAR, NCHAR, VARCHAR and NVARCHAR when we don’t specify their length in their length argument? If we omit the size of CHAR, NCHAR, VARCHAR and NVARCHAR, how they behave? In this post “CHAR, NCHAR, VARCHAR and NVARCHAR default length”, we are going to discuss the default length of CHAR, NCHAR, VARCHAR and NVARCHAR data types, if length is not specified in their argument.

I would strongly suggest to define the length of CHAR, NCHAR, VARCHAR and NVARCHAR data types as per the need. In this post, I just explain the behavior of CHAR, NCHAR, VARCHAR and NVARCHAR data types, if length is omitted, and not encouraging to omit the size of these data types.

We can define these data types as below:

char [ ( n ) ]
nchar [ ( n ) ]
varchar [ ( n | max ) ]
nvarchar [ More


Types of keys

In previous chapter, we had a good discussion on “Data Integrity” and in this chapter “Types of Keys”, we are going to discuss Keys and their types in SQL Server. Lets start this topic by defining keys”.

What is Key?

Keys are fields in a table which participate in below activities in RDBMS systems:

  1. To create relationships between two tables.
  2. To maintain uniqueness in a table.
  3. To keep consistent and valid data in database.
  4. Might help in fast data retrieval by facilitating indexes on column(s).

SQL Server supports various types of keys, which are listed below:

  1. Candidate Key
  2. Primary Key
  3. Unique Key
  4. Alternate Key
  5. Composite Key
  6. Super Key
  7. Foreign Key

Before discussing each type in brief, have a look on the below image used as an an example to define types of keys.

Types of keys

Types of keys

Lets discuss each type in detail:

Candidate Key

Candidate key is a … More


Insert into table with one identity column only 2

One of my friend asked me a question that “How we can insert values in a table with only one column which is identity, without using IDENTITY_INSERT ON?”. In this post “Insert into a table with one identity column only”, I am going to share a nice method to achieve this.

We know that using “SET IDENTITY_INSERT TableName ON” command, we can easily insert rows in an identity column. As per MSDN, we need to keep few things in mind related to “IDENTITY_INSERT ON”:

  1. Only one table can have IDENTITY_INSERT as ON in one session at a time. We must have to use “SET IDENTITY_INSERT TableName OFF” to use IDENTITY_INSERT on other tables.
  2. To execute IDENTITY_INSERT on a table, a user must own the table or has ALTER permission on the table. Only below types of user can execute IDENTITY_INSERT on a table:
    1. A member of sysadmin fixed server role
More

Delete all rows from all tables 2

Have you ever come across the problem to delete all rows from all tables available in a database? One of my colleague asked me this question that how can we delete all rows from all tables of a database. So, in this post, we are going to discuss, how we can delete all rows from all tables of a SQL Server database.

To delete all rows from a table named ProductCategory, we can simply use “DELETE FROM ProductCategory” command. But in case the table is being referred by a foreign key constraints from some tables and / or fires a trigger to insert few rows in an another table in the database, we must have to delete all related rows from child tables before we start deleting parent table rows. We also need to delete records from those table which are getting inserted rows as a result of a … More