What is the SQL query to find all records from one table based on another table?

 To find all records from one table based on conditions in another table, you typically use a JOIN or a WHERE IN clause in SQL. The specific query depends on the relationship between the two tables and what exactly you're trying to match. Here are some common scenarios:

Example Schema

• Table1: The main table you want to query.
• Table2: The table that provides the filtering conditions.

1. Using a JOIN

If there is a direct relationship (e.g., a foreign key), you can use an INNER JOIN or other types of JOINs as needed.

SELECT t1.*
FROM Table1 t1
JOIN Table2 t2
ON t1.column_name = t2.column_name;

This will return all records from Table1 where there is a matching record in Table2.

2. Using WHERE IN

If you only need to filter records based on a list of values in another table:

SELECT *
FROM Table1
WHERE column_name IN (SELECT column_name FROM Table2);

This works well when you don't need to retrieve columns from Table2 but just use it as a filter.

3. Using WHERE EXISTS

If you want to check the existence of a related record:

SELECT *
FROM Table1 t1
WHERE EXISTS (
    SELECT 1
    FROM Table2 t2
    WHERE t1.column_name = t2.column_name
);

This approach is more efficient for certain use cases, especially when Table2 has duplicate rows.

Example Use Case

If you have:

• Customers table: Contains customer details.
• Orders table: Contains customer orders.

To find all customers who have placed an order:

SELECT c.*
FROM Customers c
JOIN Orders o
ON c.customer_id = o.customer_id;

Alternatively:

SELECT *
FROM Customers
WHERE customer_id IN (SELECT customer_id FROM Orders);

Let me know if you have a specific schema or use case for a more tailored query!