Optimizing mySQL query to scale better - mysql

Question

I want to query a database to retrieve a list of names (the list of names is provided by the user in python). My criteria for looking up data for these names are the following: the results should appear in the order of the list of names the user provided, so if I say ...WHERE name = "Bob" OR name = "Alice" I want the results for Bob to come first followed by that of Alice. The second criteria is that if there is a search for a name twice, then the result should also contain it twice, so I want a way to write down ...WHERE name = 'Bob' OR name = 'Bob' so that the result also contains the rows for Bob twice.
I came up with the following query:
SELECT * FROM
(SELECT *, 1 order_position FROM table WHERE name = 'Alice'
UNION ALL
SELECT *, 2 order_position FROM table WHERE name = 'Bob'
UNION ALL
SELECT *, 3 order_position FROM table WHERE name = 'Charlie'
UNION ALL
SELECT *, 4 order_position FROM table WHERE name = 'Dan'
) r ORDER BY order_position
This query works well, but when the user submits hundreds of names and there are hundreds of UNION ALL sections, the query becomes extremely slow. Is there a way to improve the performance of the query while maintaining the two criteria mentioned before?

Answer 1

SELECT *, CASE name WHEN 'Alice' THEN 1
WHEN 'Bob' THEN 2
WHEN 'Charlie' THEN 3
WHEN 'Dan' THEN 4
END AS order_position
FROM table
WHERE name IN ('Alice', 'Bob', 'Charlie', 'Dan')
ORDER BY order_position;
or without additional column:
SELECT *
FROM table
WHERE name IN ('Alice', 'Bob', 'Charlie', 'Dan')
ORDER BY CASE name WHEN 'Alice' THEN 1
WHEN 'Bob' THEN 2
WHEN 'Charlie' THEN 3
WHEN 'Dan' THEN 4
END;
PS. For this names set ORDER BY name is enough.
How does this handle the requirement of repeating some results? –
Willem Renzema
If you need in repeating then you must convert the list to a rowset.
SELECT table.*
FROM table
JOIN ( SELECT 1 pos, 'Alice' name UNION ALL
SELECT 2 , 'Bob' UNION ALL
SELECT 3 , 'Charlie' UNION ALL
SELECT 4 , 'Bob' UNION ALL
SELECT 5 , 'Charlie' ) names USING (name)
ORDER BY names.pos

Answer 2

Somehow you have to construct the list of names with the order_position of each name.
You can do this in a query which uses UNION ALL to preserve duplicate names like this:
SELECT 'Alice' name, 1 order_position UNION ALL
SELECT 'Bob', 2 UNION ALL
SELECT 'Charlie', 3 UNION ALL
SELECT 'Dan', 4 UNION ALL
SELECT 'Alice', 1 UNION ALL
SELECT 'Bob', 2 UNION ALL
...............................
Then all you have to do is join it to the table:
SELECT t.*
FROM tablename t
INNER JOIN (
SELECT 'Alice' name, 1 order_position UNION ALL
SELECT 'Bob', 2 UNION ALL
SELECT 'Charlie', 3 UNION ALL
SELECT 'Dan', 4 UNION ALL
SELECT 'Alice', 1 UNION ALL
SELECT 'Bob', 2 UNION ALL
...............................
) n ON n.name = t.name
ORDER BY n.order_position;
In MySql 8.0+ you can use a CTE:
WITH cte(name, order_position) AS (VALUES
ROW('Alice', 1), ROW('Bob', 2), ROW('Charlie', 3),
ROW('Dan', 4), ROW('Alice', 1), ROW('Bob', 2),
...................................................
)
SELECT t.*
FROM tablename t INNER JOIN cte c
ON c.name = t.name
ORDER BY c.order_position;

Answer 3

The following will be "fast" if name is indexed:
WHERE name IN ('Alice', 'Bob', 'Charlie', 'Dan')
ORDER BY FIND_IN_SET(name, 'Alice,Bob,Charlie,Dan')
Note the syntax difference between Where and Order.
The following is likely to be somewhat slower because it cannot use any index but is simpler to code:
WHERE FIND_IN_SET(name, 'Alice,Bob,Charlie,Dan')
ORDER BY FIND_IN_SET(name, 'Alice,Bob,Charlie,Dan')
Note the restriction in FIND_IN_SET that commas cannot be used in the items.
In no case will CASE or FIND_IN_SET() use an index. (Cf "sargable")
Repeats
If, say, there are multiple "Bobs", then each of these have exactly the same effect as the above:
name IN ('Alice', 'Bob', 'Charlie', 'Bob', 'Dan')
FIND_IN_SET(name, 'Alice,Bob,Charlie,Bob,Dan')
That is, all the Bobs will be listed in the output before all the Charlies. Furthermore, no individual row is listed twice.