JIYIK CN >

This article is about using MySQL query to get data from a specific table or relation that satisfies a specific condition. To do this, the WHERE clause is used in the SQL query.

WHERE clause in SQL query

WHEREThe clause specifies the conditions for retrieving data from a single table or joining multiple tables. If the provided conditions are met, the query returns the corresponding values ​​from the table.

You can use WHEREthe clause to restrict the records and get only the records you need.

WHEREThe clause is used not only in SELECT statements, but also in UPDATE , DELETE , and other statements.

Syntax of the WHERE clause

In SQL, the following syntax is used in the WHERE clause to filter records based on a specific condition.

SELECT column1, column2, ...
FROM table_name
WHERE [condition];

You can use different relational (<, >, <=, >=, ==, !=) or logical (AND, OR, NOT) operators to create conditions.

Assume we have an Employees table which stores the employee data of an organization. The table data is as follows:

The above table shows the data of 6 employees from the Employees table. Suppose we need to select employees whose age is greater than 40, then we will use the following query:

SELECT * from Employees
WHERE Emp_Age > 40

Output:

请注意, we have used relational operators in the WHERE clause. Similarly, we can use logical and relational operators in the WHERE clause.

Using WHERE clause in MySQL to select records based on multiple conditions

We can also filter the records from the table based on multiple conditions. For this, we can use logical operators like AND and OR based on the conditions.

For example, we need to get the name and salary of employees who are under 40 years old and have a salary greater than $3000. The query for this condition would be:

SELECT Emp_Name, Emp_Salary FROM `Employees`
WHERE Emp_Age < 40 AND Emp_Salary > 3000

The result of this query will be:

As can be seen from the result, only the employees who meet both the conditions specified in the query will be selected. If we need to meet any one of the conditions, then we can use the OR operator instead of the AND operator as shown below:

SELECT Emp_Name, Emp_Salary FROM `Employees`
WHERE Emp_Age < 40 OR Emp_Salary > 3000

Now the result set will be like this:

You can see that the results contain more rows than the previous results. This is because all employees are either younger than 40 or have a salary greater than $3000.

You can achieve the same result by using the IN operator. The IN operator works the same as the OR operator, but the query is better structured.

SELECT Emp_ID,Emp_Name, Emp_Age FROM `Employees`
WHERE Emp_Name IN ("John","David")

The result set will be something like this:

In SQL query, you can see multiple ways to select multiple values ​​from a table. You can choose any one of them based on your need and desired data output.