Eliminating Sub query in MySQL - mysql

Question

I have a table in MySQL as below.
CREATE TABLE `myTable` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`employeeNumber` int(11) DEFAULT NULL,
`approveDate` date DEFAULT NULL,
`documentNumber` varchar(50) DEFAULT NULL,
PRIMARY KEY (`id`),
KEY `INDEX_1` (`documentNumber`)
) ENGINE=InnoDB;
I want to make a query if a documentNumber is approved by all employeeNumber or approved by some of employeeNumber or not approved by any employeeNumber.
I made a query as below.
SELECT T1.documentNumber,
(CASE WHEN T2.currentNum = '0' THEN '1' WHEN T2.currentNum < T2.totalNum THEN '2' ELSE '3' END) AS approveStatusNumber
FROM myTable AS T1 LEFT JOIN
(SELECT documentNumber, COUNT(*) AS totalNum, SUM(CASE WHEN approveDate IS NOT NULL THEN '1' ELSE '0' END) AS currentNum
FROM myTable GROUP by documentNumber) AS T2 ON T1.documentNumber = T2.documentNumber
GROUP BY T1.documentNumber;
This SQL works, but very slow.
I tried explain on this SQL, the result is as below.
+----+-------------+------------+-------+---------------+---------+---------+------+------+----------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+------------+-------+---------------+---------+---------+------+------+----------------------------------------------+
| 1 | PRIMARY | T1 | range | INDEX_1 | INDEX_1 | 153 | NULL | 27 | Using where; Using temporary; Using filesort |
| 1 | PRIMARY | <derived2> | ALL | NULL | NULL | NULL | NULL | 5517 | |
| 2 | DERIVED | myTable | index | NULL | INDEX_1 | 153 | NULL | 5948 | Using where |
+----+-------------+------------+-------+---------------+---------+---------+------+------+----------------------------------------------+
I think I have to eliminate sub query to improve my query.
How can I do the same thing without sub query? Or do I have another way to improve my query?

Answer 1

The expression to return currentNum could be more succinctly expressed (in MySQL) as
SUM(approveDate IS NOT NULL)
And there's no need for an inline view. This will return an equivalent result:
SELECT t.documentNumber
, CASE
WHEN SUM(t.approveDate IS NOT NULL) = 0
THEN '1'
WHEN SUM(t.approveDate IS NOT NULL) < COUNT(*)
THEN '2'
ELSE '3'
END AS approveStatusNumber
FROM myTable t
GROUP BY t.documentNumber
ORDER BY t.documentNumber