[英]select greatest element per group in querydsl
I have a sql table that looks roughly like this:我有一个 sql 表,大致如下所示:
+-----+------+-------+
| id | type | value |
+-----+------+-------+
| 1 | X | 20 |
| 2 | X | 30 |
| 3 | Y | 200 |
| 4 | Y | 500 |
| 5 | Y | 300 |
| 6 | Z | 5 |
+-----+------+-------+
For each type I want to retrieve the row with the maximum value.对于每种类型,我想检索具有最大值的行。 This is my expected result:这是我的预期结果:
+-----+------+
| id | type |
+-----+------+
| 2 | X | <-- had value = 30
| 4 | Y | <-- had value = 500
| 6 | Z | <-- had value = 5
+-----+------+
In SQL, this can be expressed as follows (given that for each type there aren't two entries with the same value, which I can rule out):在 SQL 中,这可以表示如下(假设每种类型没有两个具有相同值的条目,我可以排除):
select t1.id, t1.type from T t1
inner join (
select t2.type, max(t2.value) as max_value from T t2
group by t2.type
) on t1.type = t2.type
and t1.value = max_value
However I cannot find a way to express the same using QueryDSL (version 4).但是,我找不到使用 QueryDSL(第 4 版)表达相同内容的方法。 I tried this:我试过这个:
final JPQLQuery<Tuple> subquery = JPAExpressions
.from(q2)
.select(q2.type, q2.value.max())
.groupBy(q2.type);
final JPQLQuery<Tuple> query = JPAExpressions
.from(q1)
.select(q1.id, q1.type)
.from(q1)
.innerJoin(subquery) // <-- not allowed
.on(q1.type.eq(q2.type), q1.value.eq(q2.value.max()));
But innerJoin()
(and other join methods) only take an expression as parameter, not another query.但是innerJoin()
(和其他连接方法)只接受一个表达式作为参数,而不是另一个查询。 The same goes for from()
. from()
也是如此。
The subquery can be put into the outer query's where
clause in the form of an exists
-expression:子查询可以以exists
表达式的形式放入外部查询的where
子句中:
final JPQLQuery<Tuple> subquery = JPAExpressions
.from(q2)
.select(q2.type, q2.value.max())
.groupBy(q2.type);
final JPQLQuery<Tuple> query = JPAExpressions
.from(q1)
.select(q1.id, q1.type)
.from(q1)
.where(subquery
.having(q1.type.eq(q2.type), q1.value.eq(q2.value.max()))
.exists());
Note that this query might be quite inefficient.请注意,此查询可能效率很低。
In order to get the top 1 element, you can use:为了获得前 1 个元素,您可以使用:
JPAExpressions.selectFrom(table)
.select(table.dateTimeColumn.max())
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