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Please help, I want to connect two tables in mysql

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was right to ask for table structures and the desired result; is also correct that JOINS are the normal solution. The short guide below shows common join patterns, a small aggregation example, and the troubleshooting checks that solve most “no results” or “too many rows” problems.

An INNER JOIN returns only matching rows. Example (users one-to-many orders):

SELECT u.id, u.name, o.id AS order_id, o.total
FROM users AS u
JOIN orders AS o
  ON o.user_id = u.id;

A LEFT JOIN keeps all rows from the left table and adds matching rows from the right (NULLs when no match). Use COALESCE for defaults and GROUP BY for aggregates:

SELECT u.id, u.name, COALESCE(SUM(o.total),0) AS total_spent
FROM users AS u
LEFT JOIN orders AS o
  ON o.user_id = u.id
GROUP BY u.id, u.name;

MySQL has no native FULL OUTER JOIN; emulate it with a UNION of LEFT and RIGHT joins when needed:

SELECT u.id AS user_id, u.name, o.id AS order_id
FROM users u
LEFT JOIN orders o ON o.user_id = u.id
UNION
SELECT u.id AS user_id, u.name, o.id AS order_id
FROM users u
RIGHT JOIN orders o ON o.user_id = u.id;

Quick troubleshooting & best practices: confirm join columns have the same data type and compatible collation; index the foreign key or join column (primary key on the parent); avoid ambiguous column names by aliasing; use EXPLAIN to inspect the plan; watch one-to-many duplicates (aggregate or DISTINCT if a single row per parent is needed); NULLs, trailing spaces, or implicit type conversions commonly cause mismatches. Providing CREATE TABLE statements plus a few sample rows and the expected output makes it straightforward to produce the exact JOIN query.

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Member Avatar for pritaeas

Show you tables structures, and explain what the result should look like.

Member Avatar for Atli

I'm guessing that what you are looking for are JOINS? See 13.2.9.2. JOIN Syntax in the MySQL manual for all the details on how that works.

If you're new to this concept, the manual entry is probably not going to be very helpful. You may want to Google for some tutorials that describe this in a less technical way. This one, for example, is very good.
Understanding JOINs in MySQL and Other Relational Databases

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