MySQL groupwise MAX（）返回意外的结果 [英] MySQL groupwise MAX() returns unexpected results
问题描述
表：贷款
Loan_no金额SSS_no贷款期限
7 700.00 0104849222 2010-01-03
8 200.00 0104849222 2010-02-28
9 300.00 0119611199 2010-11-18
10 150.00 3317131410 2012-11-28
11 600.00 0104849222 2011-01-03
14 175.00 3317131410 2012-12-05
15 260.00 3317131410 2013-02-08
16 230.00 0104849222 2013-03-06
17 265.00 0119611199 2011- 04-30
18 455.00 3317131410 2013-03-10
期望的结果：
我想检索每个人获得的
的最新贷款他们的SSS号码）。
的结果应如下所示：
Loan_no金额SSS_no借款日期
16 230.00 0104849222 2013-03-06
17 265.00 0119611199 2011-04-30
18 455.00 3317131410 2013-03-10
QUERY＃1 USED： b MYSQL RESULT
$ b SELECT *
FROM loan
GROUP BY SSS_no
ORDER BY Loan_date DESC
Loan_no金额SSS_no贷款期限
10 150.00 3317131410 2012-11-28
9 300.00 0119611199 2010-11-18
7 700.00 0104849222 2010-01-03
SELECT Loan_no，Amount，SSS_no，max（Loan_date）
FROM loan
GROUP BY SSS_no
MYSQL RESULT
Loan_no金额SSS_no贷款_date
7 700.00 0104849222 2013-03-06
9 300.00 0119611199 2011-04-30
10 150.00 3317131410 2013-03-10
任何人都可以帮我解决问题吗？感谢。
MySQL参考资料提出了几种解决方法。最简单的是一个子查询：
$ pre code $ SELECT
FROM loan l1
WHERE loan_date =（SELECT MAX（ l2.loan_date）
FROM loan l2
WHERE l1.sss_no = l2.sss_no）;
鉴于这种类型的子查询可能性能不佳，他们还建议使用 JOIN Mahmoud Gamal的回答）：
pre code> SELECT l1.loan_no，l1.amount，l1.sss_no，l1.loan_date
FROM贷款l1
JOIN（
SELECT loan_no，MAX（loan_date）AS loan_date
FROM loan
GROUP BY sss_no）AS l2
ON l1.loan_date = l2.loan_date AND l1.sss_no = l2.sss_no;
第三种选择是：
SELECT l1.loan_no，l1.amount，l1.sss_no，l1.loan_date
FROM loan l1
LEFT JOIN贷款l2 ON l1.sss_no = l2.sss_no AND l1。 loan_date< l2.loan_date
WHERE l2.sss_no IS NULL;
LEFT JOIN 的工作原理是当 l1.loan_date 处于最大值时，后面会出现 l2.loan_date ，所以l2行值将会是NULL。
所有这些应该有相同的输出，但可能会有不同的表现。
TABLE: LOAN
Loan_no Amount SSS_no Loan_date
7 700.00 0104849222 2010-01-03
8 200.00 0104849222 2010-02-28
9 300.00 0119611199 2010-11-18
10 150.00 3317131410 2012-11-28
11 600.00 0104849222 2011-01-03
14 175.00 3317131410 2012-12-05
15 260.00 3317131410 2013-02-08
16 230.00 0104849222 2013-03-06
17 265.00 0119611199 2011-04-30
18 455.00 3317131410 2013-03-10
DESIRED RESULTS:
I would want to retrieve the latest loan availed off by each person (identified by their SSS number). The results should be as follows:
Loan_no Amount SSS_no Loan_date
16 230.00 0104849222 2013-03-06
17 265.00 0119611199 2011-04-30
18 455.00 3317131410 2013-03-10
QUERY # 1 USED:
SELECT *
FROM loan
GROUP BY SSS_no
ORDER BY Loan_date DESC
MYSQL RESULT
Loan_no Amount SSS_no Loan_date
10 150.00 3317131410 2012-11-28
9 300.00 0119611199 2010-11-18
7 700.00 0104849222 2010-01-03
QUERY # 2 USED:
SELECT Loan_no, Amount, SSS_no, max(Loan_date)
FROM loan
GROUP BY SSS_no
MYSQL RESULT
Loan_no Amount SSS_no Loan_date
7 700.00 0104849222 2013-03-06
9 300.00 0119611199 2011-04-30
10 150.00 3317131410 2013-03-10
Can anybody help me with my problem? Thanks.
The MySQL reference suggests several ways to solve this. The simplest is a subquery:
SELECT *
FROM loan l1
WHERE loan_date=(SELECT MAX(l2.loan_date)
FROM loan l2
WHERE l1.sss_no = l2.sss_no);
Given that this type of subqueries potentially have bad performance, they also suggest using a JOIN (essentially Mahmoud Gamal's answer):
SELECT l1.loan_no, l1.amount, l1.sss_no, l1.loan_date
FROM loan l1
JOIN (
SELECT loan_no, MAX(loan_date) AS loan_date
FROM loan
GROUP BY sss_no) AS l2
ON l1.loan_date = l2.loan_date AND l1.sss_no = l2.sss_no;
A third option is:
SELECT l1.loan_no, l1.amount, l1.sss_no, l1.loan_date
FROM loan l1
LEFT JOIN loan l2 ON l1.sss_no = l2.sss_no AND l1.loan_date < l2.loan_date
WHERE l2.sss_no IS NULL;
The LEFT JOIN works on the basis that when l1.loan_date is at its maximum value, there is later l2.loan_date, so the l2 row values will be NULL.
All these should have the same output, but likely differ in performance.
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