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WHERE Clause

The WHERE clause filters query results. It restricts the rows returned based on specified conditions. Position this clause directly after FROM.

SELECT column_name FROM table_name WHERE condition;

The condition is an expression evaluating to true, false, or unknown. Only rows where the condition evaluates to true are included in the result set.

WHERE Clause Example

Consider a users table. To retrieve only active users:

SELECT DISTINCT username, email, active FROM users WHERE active=true;

Output demonstrates results filtered by the active= true condition.

username	email	active
shinobi	b@javateamsix.com	1
tony	t@javateamsix.com	1

Conversely, retrieve inactive users by setting active=false:

SELECT DISTINCT username, email, active FROM users WHERE active=false;

Output reflects rows where active is false (0).

username	email	active
Java Team Six	d@javateamsix.com	0

To select users with a specific username, like 'shinobi':

SELECT username, email, active FROM users WHERE username='shinobi';

Multiple entries match the username 'shinobi'.

username	email	active
shinobi	b@javateamsix.com	1
shinobi	b@javateamsix.com	1

Operators

Comparison operators define conditions within the WHERE clause.

• = : Equal to
• != : Not equal to
• > : Greater than
• >= : Greater than or equal to
• < : Less than
• <= : Less than or equal to

AND Keyword

Combine multiple conditions with the AND keyword. The AND operator returns rows where all conditions are true.

Example: Select active users with username 'shinobi'.

SELECT  * FROM users WHERE username='shinobi' AND active=true;

Output shows rows matching both the username and active status.

id	username	about	birthday	active	email
2	shinobi	NULL	NULL	1	b@javateamsix.com
5	shinobi	NULL	NULL	1	b@javateamsix.com

If no rows satisfy all conditions, the result is empty.

SELECT * FROM users WHERE username='shinobi' AND active=false;

-- Output: Empty set.

OR Keyword

Combine conditions with the OR keyword. The OR operator returns rows where any condition is true. Use OR to fetch data matching one criterion or another.

Example: Select users with username 'shinobi' OR where active is false.

SELECT * FROM users WHERE username='shinobi' OR active=false;

Output includes rows where either condition is met.

id	username	about	birthday	active	email
2	shinobi	NULL	NULL	1	b@javate amsix.com
3	Java Team Six	NULL	NULL	0	d@javateamsix.com
5	shinobi	NULL	NULL	1	b@javateamsix.com
6	Java Team Six	NULL	NULL	0	d@javateamsix.com

LIKE Operator

The LIKE operator performs pattern matching. It is typically used with wildcard characters (% for any sequence of characters, _ for any single character).

Example: Select users where the username contains the letter 'y'.

SELECT * FROM users  WHERE username LIKE '%y%';

Output includes usernames containing 'y'.

id	username	about	birthday	active	email
2	shinobi	NULL	NULL	1	b@javateamsix.com
4	tony	NULL	NULL	1	t@javateamsix.com

IN Operator

The IN operator checks if a column's value matches any value in a specified list. This simplifies conditions that would otherwise require multiple OR statements.

Example: Select users with username 'shinobi' or ' Java Team Six'.

SELECT * FROM users WHERE username IN ('shinobi', 'Java Team Six');

Output shows all rows where the username is in the provided list.

id	username	about	birthday	active	email
2	shinobi	NULL	NULL	1	b@javateamsix.com
3	Java Team Six	NULL	NULL	0	d@javateamsix.com
5	shinobi	NULL	NULL	1	b@javateamsix .com
6	Java Team Six	NULL	NULL	0	d@javateamsix.com

IS Operator

Use the IS operator specifically for checking NULL values . The = operator cannot be used for this purpose.

To find rows where about is NULL:

SELECT * FROM users WHERE about IS NULL;

To find rows where about is not NULL:

SELECT * FROM users WHERE about IS NOT NULL;

BETWEEN Operator

The BETWEEN operator selects values within a defined range. This range is inclusive of the start and end values. Applicable to numbers, text, and dates.

Example: Select users with an ID between 3 and 6 (inclusive).

SELECT * FROM users WHERE id BETWEEN 3 AND 6;

Output displays rows where id is within the specified range.

id	username	about	birthday	active	email
3	Java Team Six	NULL	NULL	0	d@javateamsix.com
5	shinobi	NULL	NULL	1	b@javateamsix.com
6	Java Team Six	NULL	NULL	0	d@javateamsix.com

Summary

The WHERE clause and its associated operators are fundamental tools for precisely selecting data subsets, essential for efficient database interaction from Java applications.