我们的一个 MySQL 语句有问题。
基本上，以下语句需要 5 秒才能运行。我们已经诊断它归结为两个 select 语句的连接。当 select 语句单独运行时，它们只需要 0.2 秒，但与 JOIN 结合使用时需要 5 秒。
有什么你可以看到我们做错了，或者你能看到更好的方法吗？
索引已添加到连接中包含的所有列，但它不影响速度
SELECT temp_4.primaryid, temp_1.`subjectID` , temp_4.`testOccasionID`
,`studyNumbers` ,`testDate`
FROM (
SELECT * FROM (
SELECT primarys.primaryid , q_1 AS `subjectID` , q_2 AS `studyNumbers` FROM
primarys LEFT OUTER JOIN questions_1_100 ON primarys.primaryid =
questions_1_100.primaryid WHERE 0 = 0 AND q_1 IS NOT NULL GROUP BY primaryid) AS
maintable_1
GROUP BY `subjectID` ) AS temp_1
JOIN
(SELECT * FROM
(SELECT primarys.primaryid , q_1 AS `subjectID` , q_4 AS `testOccasionID` ,
DATE_FORMAT(q_5, '%m/%d/%Y') AS `testDate` FROM primarys LEFT OUTER JOIN
questions_1_100 ON primarys.primaryid = questions_1_100.primaryid WHERE 0 = 0 AND
q_1 IS NOT NULL AND q_4 IS NOT NULL GROUP BY primaryid) AS maintable_4
GROUP BY `subjectID` ,`testOccasionID` ) AS temp_4
ON temp_1.`subjectID` = temp_4.`subjectID`
表定义：
CREATE TABLE primarys
( primaryid BIGINT(20) NOT NULL AUTO_INCREMENT,
dateinserted DATETIME,
datemodified DATETIME,
useridinserted BIGINT(20),
useridmodified BIGINT(20),
locked VARCHAR(1) NOT NULL DEFAULT 0,
primaryquestionlinks TEXT,
PRIMARY KEY (primaryid),
FOREIGN KEY (useridinserted) REFERENCES users (userid) ON UPDATE CASCADE ON DELETE SET NULL,
FOREIGN KEY (useridmodified) REFERENCES users (userid) ON UPDATE CASCADE ON DELETE SET NULL ) ENGINE=InnoDB;
CREATE TABLE questions_1_100
( primaryid BIGINT(20) NOT NULL,
q_1 BIGINT(20),
q_2 VARCHAR(50),
q_3 BIGINT(20),
q_4 BIGINT(20),
q_5 DATE,
PRIMARY KEY (primaryid),
FOREIGN KEY (primaryid) REFERENCES primarys (primaryid) ON UPDATE CASCADE ON DELETE CASCADE ) ENGINE=InnoDB;
单个行军主体数据如下——顺序为primaryid,q_1,q_2,q_4,q_5：
1 1 01001 NULL NULL
7286 1 NULL 1 1997-12-18
7287 1 NULL 2 1998-02-25
所需的输出是：
7286 1 01001 1 1997-12-18
7287 1 01001 2 1998-02-25
非常感谢
更进一步……如果 q_1 和 q_4 在两个单独的表中会怎样。比如下面的表结构。我能想到的唯一方法是添加左外连接和几个子查询？
CREATE TABLE primarys
( primaryid BIGINT(20) NOT NULL AUTO_INCREMENT,
PRIMARY KEY (primaryid));
CREATE TABLE questions_1_100
( primaryid BIGINT(20) NOT NULL,
q_1 BIGINT(20),
q_2 VARCHAR(50),
PRIMARY KEY (primaryid));
CREATE TABLE questions_101_200
( primaryid BIGINT(20) NOT NULL,
q_4 BIGINT(20),
q_5 DATE,
PRIMARY KEY (primaryid));
INSERT INTO primarys values (1);
INSERT INTO primarys values (7286);
INSERT INTO primarys values (7287);
INSERT INTO questions_1_100 VALUES (1,'1','01001');
INSERT INTO questions_1_100 VALUES (7286,'1','');
INSERT INTO questions_1_100 VALUES (7287,'1','');
INSERT INTO questions_101_200 VALUES (7286,'1','1997-12-18');
INSERT INTO questions_101_200 VALUES (7287,'2','1998-02-25');