Merge the query results,optimize? - mysql

Question

mysql,two tables：test(one) and review(many).
My goal:from review for one in the corresponding number
SELECT t.ID,t.TITLE,COALESCE(COUNT(r.ID),0) `count`
FROM `test` t
LEFT OUTER JOIN review r
ON t.ID = r.REVIEW_OBJ_ID
WHERE r.REVIEW_TYPE = '4'
ORDER BY `count` DESC;
Output:
ID TITLE count
402884f657e0a6d20157e0a82cc90000 brother 2
test table(A small portion of the data)
SELECT t.ID,t.TITLE
FROM `test` t;
Output:
ID TITLE
40284c8157ad8e7d0157ad8f86880000 1234567890123456789012345
402884f657e0a6d20157e0a82cc90000 brother
402884f657e0a6d20157e11967a20036 fg
402884f657e51eff0157e54cd8610004 AAA
402884f657e652fb0157e65642750000 BBB
0000000057f4dc900157f4ea9edd0000 VVV
00000000580065c5015800746d750000 CCC
00000000580065c501581d9f04f0000b TTT
And I want get this:
ID TITLE count
402884f657e0a6d20157e0a82cc90000 brother 2
402884f657e652fb0157e65642750000 BBB 0
00000000580065c501581d9f04f0000b TTT 0
402884f657e0a6d20157e11967a20036 fg 0
0000000057f4dc900157f4ea9edd0000 VVV 0
40284c8157ad8e7d0157ad8f86880000 1234567890123456789012345 0
402884f657e51eff0157e54cd8610004 AAA 0
00000000580065c5015800746d750000 CCC 0
so,I tried this and it worked:
SELECT t.ID,t.TITLE, COALESCE(r.c,0) `count`
FROM `test` t
LEFT OUTER JOIN
(
SELECT r.REVIEW_OBJ_ID obj_id, COUNT(r.ID) c
FROM review r,`test` t
WHERE r.REVIEW_TYPE = '4'
AND t.ID = r.REVIEW_OBJ_ID
) r ON r.obj_id = t.ID
ORDER BY `count` DESC;
But I have two questions:
It feels I can use one-time select to found out result,but I use two-times select.Can I optimize it?
Add a count(redundant) in the test table fields, whether it is a better choice.
/REVIEW_TYPE and REVIEW_OBJ_ID decide which object is reviewed,just like I use "REVIEW_TYPE='4'" to contact the test table/
drop table if exists user_doctor_review;
create table review
(
ID varchar(64) not null,
USER_ID varchar(64),
DOCTOR_ID varchar(64),
REVIEW_TYPE varchar(1),
REVIEW_OBJ_ID varchar(64),
SERVICE_SCORE int(6),
REVIEW_CONTENT varchar(600),
REVIEW_TIME datetime,
POID varchar(64),
IS_ANONYMITY varchar(1),
CHECKED_STATUS varchar(1),
STATUS varchar(1),
REPLY_CONTENT varchar(600),
REPLY_TIME datetime,
DOCTOR_IS_READ varchar(1),
primary key (ID)
);

Answer 1

Yes you can do it with one SELECT statement with this query
SELECT test.ID, test.TITLE, count(review.ID) as count from test
left join review on test.ID = review.REVIEW_OBJ_ID
where review.REVIEW_TYPE = 4 or review.ID is null
group by test.ID
I have created a SQL Fiddle here: http://sqlfiddle.com/#!9/c6a178/15
Explanation:
The key points are:
or review.ID is null
because it will make the query list the tests with no reviews, and
group by test.ID
this will get the correct review counts related to the test.
Results:
ID TITLE count
-------------------------------- ------------------------- -----
0000000057f4dc900157f4ea9edd0000 VVV 0
00000000580065c5015800746d750000 CCC 0
00000000580065c501581d9f04f0000b TTT 1
40284c8157ad8e7d0157ad8f86880000 1234567890123456789012345 0
402884f657e0a6d20157e0a82cc90000 brother 2
402884f657e0a6d20157e11967a20036 fg 0
402884f657e51eff0157e54cd8610004 AAA 0
402884f657e652fb0157e65642750000 BBB 0