When and where to use Where clause in SQL

 In this post we will learn when and where o use where clause in SQL. The SQL WHERE clause is used to filter and retrieve specific rows from a database table based on certain conditions. It allows you to specify criteria that must be met for a row to be included in the result set of a SQL query. Here are some common scenarios where you can use the WHERE clause:

Retrieving specific rows or filter particular row:

 You can use the WHERE clause to fetch rows  from table that match specific conditions.

 For example, if you have a table of employees and you want to retrieve all employees who are in the "Sales" department, you can use a WHERE clause like this:

Example

SELECT * FROM employees WHERE department = 'Sales';

This query will return all rows from the "employees" table where the "department" column has the value "Sales." The "*" is a wildcard that selects all columns. If you only want specific columns, you can replace "*" with the column names you desire, separated by commas.

Filtering based on multiple conditions

The WHERE clause allows you to combine multiple conditions using logical operators such as AND and OR. 

For instance, if you want to retrieve employees who are in the "Sales" department and have a salary greater than $50,000, you can use the WHERE clause like this:

Example

SELECT * FROM employees WHERE department = 'Sales', AND salary > 50000;

In this example, the query selects all rows from the "employees" table where the department is "Sales" AND the salary is greater than 50000. You can add as many conditions as needed using the logical operators to create more complex filters.

If you want to filter based on multiple conditions where either condition can be true, you can use the "OR" operator. Here's an example:

Example

SELECT * FROM employees WHERE department = 'Sales' OR department = 'Marketing';

This query selects all rows from the "employees" table where the department is either "Sales" OR "Marketing."

Searching with patterns

To search for patterns within your data, you can use the SQL "LIKE" operator along with wildcard characters. The "%" character represents zero or more characters, and the "_" character represents a single character. Here's an example:

Example

SELECT * FROM employees WHERE name LIKE 'J%';

This query selects all rows from the "employees" table where the name starts with the letter 'J'. The "%" wildcard after the 'J' allows for any number of characters to follow.If you want to search for names that end with a specific pattern, you can use the name with this pattern '%son' also you can  use the "%" wildcard on both sides of the pattern to search for names that contain a specific sequence of characters anywhere within them like this '%doe%.

Filtering based on range or comparisons

 The WHERE clause can be used to filter rows based on ranges or comparisons of column values.

 For example, if you want to retrieve all products with a price between $10 and $50, you can use the WHERE clause like this:

Example

SELECT * FROM products WHERE price BETWEEN 10 AND 50;

This query selects all rows from the "products" table where the price is greater than or equal to $10 and less than or equal to $50. Adjust the column name ("price") and the table name ("products") according to your specific dataset.

Filtering NULL or NOT NULL values

 You can use the WHERE clause to filter rows based on the presence or absence of NULL values in columns. 

For example, if you want to retrieve all products who have a NULL value for their price, you can use the WHERE clause like this:

Example

SELECT * FROM products WHERE price IS NULL;

This query will select all rows from the "products" table where the "price" column contains a NULL value. You will get the products that do not have a specified price.

On the other hand, if you want to retrieve products that have a non-NULL value for the "price" column, you can use the following query:

Example

SELECT * FROM products WHERE price IS NOT NULL;

This query will select all rows from the "products" table where the "price" column does not contain a NULL value. You will get the products that have a specified price.

Conclusion:

The WHERE clause in SQL is used to filter rows from a table based on specific conditions. It allows you to retrieve only the rows that meet the specified criteria, whether it's based on values, range, comparisons, patterns, or NULL values. The WHERE clause helps you to refine your query results and retrieve the desired subset of data from a table