Show only one result in sql LEFT JOIN - mysql

Question

Table A
id | kind | item_name
---+-------+----------
1 | one | item_1
2 | one | item_2
Table B
tableA_id | kind | price
----------+-------+------
1 | all | $10
1 | one | $2
then i have a query like this
SELECT a.id, a.kind, a.name, b.price FROM table_a a
LEFT JOIN table_b b ON b.tableA_id = a.id
AND (a.kind = b.kind OR a.kind = 'all')
WHERE a.id = 1
Basically i want to show the price of all if the kind does not exist in table b, however if the kind is exist then show only the price one as example. but my query show both all and one.
The result of the about query is like this:
id | kind | item_name | price
---+-------+-----------+------
1 | all | item_1 | $10
1 | one | item_1 | $2
I expect this
id | kind | item_name | price
---+-------+-----------+------
1 | one | item_1 | $2

Answer 1

Update:
Basically i want to show the price of all if the kind does not exist
in table b, however if the kind is exist then show only the price one
as example.
For this, you can use CASE expression to do this:
SELECT a.id, a.kind, a.item_name,
CASE WHEN b.price IS NULL
THEN (SELECT b2.Price
FROM table_b AS b2
WHERE b2.kind = 'all')
ELSE b.price
END AS PRice
FROM table_a a
LEFT JOIN table_b b ON b.tableA_id = a.id
AND b.kind = 'one'
WHERE a.id = 1
updated fiddle demo

Answer 2

This should work in theory, but you're doing two joins:
SELECT a.id, a.kind, a.name, COALESCE(b1.price, b2.price) AS price
FROM table_a AS a
LEFT JOIN table_b AS b1
ON b1.tableA_id = a.id
AND b1.kind = a.kind
LEFT JOIN table_b AS b2
ON b1.tableA_id = a.id
AND b1.kind = 'all'
WHERE a.id = 1;
Could be more efficient. Basically you're first finding whether there is something with matching kind price and fallback to 'all' price.
Also - if table_b is always going to have item price for 'all' you could change that left join to inner join.

Answer 3

To pick single row for your scenario you would need another self join in your query with additional comparison check to pick single row if another kind exist in table b else show a row with kind = all
SELECT a.id, a.kind, a.item_name, b.price
FROM table_a a
INNER JOIN table_b b ON b.tableA_id = a.id
LEFT JOIN table_b b1 ON b.tableA_id = b1.tableA_id
AND b.kind < b1.kind
WHERE b1.tableA_id IS NULL;
DEMO where kind exist
DEMO where kind doesn't exist
Make sure the keyword is 'all' starts with a then this b.kind < b1.kind can work properly because if you do
select 'all' < 'one'; // will return true
select 'all' > 'one'; // will return false
Demo