SQL WHERE
Summary: in this tutorial, you will learn how to use the SQL WHERE
clause to filter rows based on specified conditions.
Introduction to SQL WHERE
clause
To select certain rows from a table, you use a WHERE
clause in the SELECT
statement. The following illustrates the syntax of the WHERE
clause in the SELECT
statement:
SELECT
column1, column2, ...
FROM
table
WHERE
condition;
The WHERE
the clause appears immediately after the FROM
clause. The WHERE
the clause contains one or more logical expressions that evaluate each row in the table. If a row that causes the condition evaluates to true, it will be included in the result set; otherwise, it will be excluded.
Note that SQL has three-valued logic which are TRUE, FALSE, and UNKNOWN. It means that if a row causes the condition to evaluate to FALSE or NULL, the row will not be returned.
Note that the logical expression that follows the WHERE
clause is also known as a predicate. You can use various operators to form the row selection criteria used in the WHERE
clause.
The following table shows the SQL comparison operators:
Operator | Meaning |
---|---|
= | Equal to |
<> (!=) | Not equal to |
< | Less than |
> | Greater than |
<= | Less than or equal |
>= | Greater than or equal |
To form a simple expression, you use one of the operators above with two operands that can be either column name on one side and a literal value on the other, for example:
salary > 1000
It asks a question: “Is salary greater than 1000?”.
Or you can use column names on both sides of an operator such as:
min_salary < max_salary
This expression asks another question: “Is the min salary less than the max salary?”.
The literal values that you use in an expression can be numbers, characters, dates, and times, depending on the format you use:
- Number: use a number that can be an integer or a decimal without any formatting e.g., 100, 200.5
- Character: use characters surrounded by either single or double quotes e.g., “100”, “John Doe”.
- Date: use the format that the database stores. It depends on the database system e.g., MySQL uses
'yyyy-mm-dd'
format to store the date data. - Time: use the format that the database system uses to store the time. For example, MySQL uses
'HH:MM:SS'
to store time data.
Besides the SELECT
the statement, you can use the WHERE
clause in the UPDATE
or DELETE
statement to specify which rows to be updated or deleted.
SQL WHERE
examples
We will use the employees
table to demonstrate how to select data from the table using the WHERE
clause.
SQL WHERE
the clause with numeric comparison examples
The following query finds employees who have salaries greater than 14,000 and sorts the result set based on the salary in descending order.
SELECT employee_id, first_name, last_name, salary FROM employees WHERE salary > 14000 ORDER BY salary DESC;
The following query finds all employees who work in the department id 5.
SELECT
employee_id,
first_name,
last_name,
department_id
FROM
employees
WHERE
department_id = 5
ORDER BY
first_name;
SQL WHERE
the clause with characters example
SQL is case-insensitive. However, when it comes to the values in the comparisons, it is case-sensitive. For instance, the following query finds the employee whose last name is Chen
.
SELECT
employee_id,
first_name,
last_name
FROM
employees
WHERE
last_name = 'Chen';
However, if you use CHEN
or chen
, no row will be returned.
SQL WHERE
the clause with dates examples
To get all employees who joined the company after January 1st, 1999
, you use the following query:
SELECT
employee_id,
first_name,
last_name,
hire_date
FROM
employees
WHERE
hire_date >= '1999-01-01'
ORDER BY
hire_date DESC;
If you want to find the employees who joined the company in 1999, you have several ways:
- Use the
YEAR
function to get the year data from thehire_date
column and use the equal to (=) operator to form the expression. - Use two expressions with the
AND
operator. - Use the
BETWEEN
operator.
The following statement illustrates the first way:
SELECT
employee_id,
first_name,
last_name,
hire_date
FROM
employees
WHERE
YEAR (hire_date) = 1999
ORDER BY
hire_date DESC;
In this tutorial, we have shown you how to use the SQL WHERE
clause to filter data based on a specified condition.
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