MySQL left join with 'b's limit - php

Question

First of all, i'm sorry to my wrong english.
I have one question for mysql query join with b.limit
show below two table
Table 1 : Members
id INT NOT NULL PRIMARY KEY
user_name VARCHAR(100) NOT NULL
...
Table 2 : Members_opt
id INT NOT NULL PRIMARY KEY
members_id VARCHAR(100) NOT NULL
category varchar(10) NOT NULL
...
and one user have multiple columns of Members_opt.
and Members.id = Members_opt.members_id.
I want make this query
SELECT * FROM Members a, Members_opt b WHERE a.id = b.members_id;
But this query makes below result.
id | user_name | category
01 | John | cat
01 | John | dog
01 | John | bird
02 | olion | cat
03 | jenny | dog
I want if result have same id value, just add "limit 0,1" to category
so i want this result.
01 | John | cat
02 | olion | cat
03 | jenny | dog
How can I achieve this?

Answer 1

The JOIN operation presents all possible combinations of rows in the two tables. So, if John has a dog, cat, and bird, and you do an ordinary JOIN, you'll get three rows for John.
(SELECT .. FROM a,b WHERE a.something=b.something is a JOIN. It uses old-fashioned syntax but is still a JOIN).
To meet your requirement, you need a way to get a virtual table (a subquery) containing just one pet per member.
SELECT members_id,
MAX(category) category
FROM Members_opt
GROUP BY members_id
will do that. It selects the category (pet) with the name coming last in the alphabet. It returns at most one row per value of members_id.
Then you JOIN that virtual table (subquery) to your other table.
SELECT a.id, a.user_name, b.category
FROM Members a
JOIN (
SELECT members_id,
MAX(category) category
FROM Members_opt
GROUP BY members_id
) b ON a.id = b.members_id
This will return one row for each member showing the chosen category. If a member has no categories (no pets) this query will suppress that member's row. If you want your query to show members having no pets, use LEFT JOIN.
Notice that this query
SELECT * /* incorrect: nonstandard, unpredictable */
FROM Members a, Members_opt b
WHERE a.id = b.members_id
GROUP BY a.id
misuses a nonstandard extension to MySQL's implementation of GROUP BY. It won't work in newer versions of MySQL, but rather will get an error. It will, if it does work at all, return unpredictable results. Read this. https://dev.mysql.com/doc/refman/5.7/en/group-by-handling.html
Returning unpredictable results is worse than returning random results. When you misuse this GROUP BY hack in MySQL, each subsequent use of the query returns the same results, until it doesn't.