email regex validation leetcode sql solution and EXCEPT sql query in postgres

Published: 21 June 2024
on channel: CodeWis Technologies by Nuhman Paramban

The given SQL query selects all columns from a table named users where the email addresses in the mail column match a specific pattern. Here's a detailed breakdown of each part of the query:

Query Breakdown
sql
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SELECT *
FROM users AS u
WHERE u.mail REGEXP '^[a-zA-Z][a-zA-Z0-9._-]*@leetcode.com$';
SELECT *:

This part of the query selects all columns from the result set.
FROM users AS u:

This specifies the table from which to retrieve the data. The table is users, and it is given an alias u for use in the query.
WHERE u.mail REGEXP '^[a-zA-Z][a-zA-Z0-9._-]*@leetcode.com$':

This is the condition that filters the rows based on the email address pattern in the mail column.
Regular Expression Breakdown
The regular expression '^[a-zA-Z][a-zA-Z0-9._-]*@leetcode.com$' is used to match email addresses with the following pattern:

^: Asserts the position at the start of the string.
[a-zA-Z]: Matches any single letter (either uppercase or lowercase) from A to Z. This ensures the email starts with a letter.
[a-zA-Z0-9._-]*: Matches zero or more occurrences of letters (A-Z, a-z), digits (0-9), periods (.), underscores (_), or hyphens (-). This part matches the local part of the email address after the first character.
@leetcode.com: Matches the literal string @leetcode.com, ensuring that the email domain is leetcode.com.
$: Asserts the position at the end of the string.
Putting It All Together
The query selects all rows from the users table where the mail column contains an email address that:

Starts with a letter (A-Z or a-z).
Is followed by any combination of letters, digits, periods, underscores, or hyphens.
Ends with the domain @leetcode.com.
Example Matches
Some examples of email addresses that would match this pattern:

[email protected]
[email protected]
[email protected]
Examples of email addresses that would not match:

[email protected] (starts with a digit)
[email protected] (wrong domain)
alice@leetcode (domain is incomplete)
[email protected] (extra domain parts)
Use Case
This query is particularly useful when you want to filter and retrieve users from the users table who have email addresses under the specific domain leetcode.com and conform to a specific format, ensuring proper validation of the email format.

The EXCEPT operator in PostgreSQL is used to return all rows from the first SELECT statement that are not present in the second SELECT statement. Here's a detailed explanation of the query:

SELECT Name
FROM Employees
EXCEPT
SELECT Name
FROM Contractors;

Explanation
Bob and Charlie are employees who are not listed as contractors.
Alice is an employee, but she is also a contractor, so she is excluded from the final result.
Use Case
This query is particularly useful when you want to find entities (in this case, employee names) that exist in one table but not in another. In this example, it helps to identify employees who are not also contractors.

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