SQL: LIKE Condition

The SQL LIKE condition allows you to use wildcards in the SQL WHERE clause of an SQL statement. This allows you to perform pattern matching. The SQL LIKE condition can be used in any valid SQL statement - SQL SELECT statement, SQL INSERT statement, SQL UPDATE statement, or SQL DELETE statement.

The patterns that you can choose from are:

• % allows you to match any string of any length (including zero length)
• _ allows you to match on a single character

SQL LIKE Condition - Using % wildcard example

Let's explain how the % wildcard works in the SQL LIKE condition. We are going to try to find all of the suppliers whose name begins with 'Hew'.

SELECT * FROM suppliers
WHERE supplier_name like 'Hew%';

You can also using the % wildcard multiple times within the same string. For example,

SELECT * FROM suppliers
WHERE supplier_name like '%bob%';

In this SQL LIKE condition example, we are looking for all suppliers whose name contains the characters 'bob'.

You could also use the SQL LIKE condition to find suppliers whose name does not start with 'T'.

For example:

SELECT * FROM suppliers
WHERE supplier_name not like 'T%';

By placing the not keyword in front of the SQL LIKE condition, you are able to retrieve all suppliers whose name does not start with 'T'.

Normalization of Database.

Description of Normalization

Normalization is the process of organizing data in a database. This includes creating tables and establishing relationships between those tables according to rules designed both to protect the data and to make the database more flexible by eliminating redundancy and inconsistent dependency. 

Redundant data wastes disk space and creates maintenance problems. If data that exists in more than one place must be changed, the data must be changed in exactly the same way in all locations. A customer address change is much easier to implement if that data is stored only in the Customers table and nowhere else in the database. 

What is an "inconsistent dependency"? While it is intuitive for a user to look in the Customers table for the address of a particular customer, it may not make sense to look there for the salary of the employee who calls on that customer. The employee's salary is related to, or dependent on, the employee and thus should be moved to the Employees table. Inconsistent dependencies can make data difficult to access because the path to find the data may be missing or broken. 

There are a few rules for database normalization. Each rule is called a "normal form." If the first rule is observed, the database is said to be in "first normal form." If the first three rules are observed, the database is considered to be in "third normal form." Although other levels of normalization are possible, third normal form is considered the highest level necessary for most applications. 

As with many formal rules and specifications, real world scenarios do not always allow for perfect compliance. In general, normalization requires additional tables and some customers find this cumbersome. If you decide to violate one of the first three rules of normalization, make sure that your application anticipates any problems that could occur, such as redundant data and inconsistent dependencies.