我有一个表用ISO 4217 values货币（有6行，ID，国家，Currency_Name，Alphabetic_code，Numeric_Code，Minor_Unit）。
我需要获取4 most used currencies的一些数据，我的“纯”SQL查询如下：
select distinct currency_name, alphabetic_code, numeric_code
from currency
where ALPHABETIC_CODE IN ('USD','EUR','JPY','GBP')
order by currency_name;
它返回一个包含我需要的数据的4行表。到现在为止还挺好。
现在我必须将它转换为我们的JPA xml文件，问题就开始了。我想要的查询是这样的：
SELECT DISTINCT c.currencyName, c.alphabeticCode, c.numericCode
FROM Currency c
WHERE c.alphabeticCode IN ('EUR','GBP','USD','JPY')
ORDER BY c.currencyName
对于具有某些货币的每个国家/地区，这将返回一行列表（就好像查询中没有“DISTINCT”）。我正在摸不着原因。所以问题是：
1）如何使此查询返回“纯”SQL给我的内容？
2）为什么这个查询似乎忽略了我的“DISTINCT”条款？这里有一些我不知道的东西，我什么也得不到。发生了什么，我没有得到什么？
编辑：嗯，这更令人讨厌。不知何故，JPA查询按预期工作（返回4行）。我试过这个（因为我需要更多信息）：
SELECT DISTINCT c.currencyName, c.alphabeticCode, c.numericCode, c.minorUnit, c.id
FROM Currency c
WHERE c.alphabeticCode IN ('EUR','GBP','USD','JPY')
ORDER BY c.currencyName
似乎ID正在弄乱一切，因为从查询中删除它会返回4行表。括号括号是没用的。
顺便说一句，我们正在使用eclipse链接。
您遇到的问题是当您尝试检索列列表时（c.currencyName, c.alphabeticCode, c.numericCode, c.minorUnit, c.id)）
我相信“id”列对于db表中的每个记录都是唯一的，因此您可以在其他列(c.currencyName, c.alphabeticCode, c.numericCode, c.minorUnit)中获取重复项。
所以在这种情况下，DISTINCT在整行上运行，而不是在特定列上运行。如果要获取唯一名称，请仅选择该列。
如果你想在多个列上运行distinct，你可以做这样的事情，例如使用GROUP BY来使用c.currencyName, c.alphabeticCode进行查找
SELECT DISTINCT c.currencyName, c.alphabeticCode, c.numericCode,c.id
FROM Currency c
WHERE c.alphabeticCode IN ('EUR','GBP','USD','JPY') GROUP BY c.currencyName, c.alphabeticCode
ORDER BY c.currencyName
要回答你的问题，你写的JPQL查询就可以了：
SELECT DISTINCT c.currencyName, c.alphabeticCode, c.numericCode
FROM Currency c
WHERE c.alphabeticCode IN ('EUR','GBP','USD','JPY')
ORDER BY c.currencyName
它应该转换为您期望的SQL语句：
select distinct currency_name, alphabetic_code, numeric_code
from currency
where ALPHABETIC_CODE IN ('USD','EUR','JPY','GBP')
order by currency_name;
现在，正如我在this article中所解释的，根据底层的JPQL或Criteria API查询类型，DISTINCT在JPA中有两个含义。
对于返回标量投影的标量查询，如下面的查询：
List<Integer> publicationYears = entityManager
.createQuery(
"select distinct year(p.createdOn) " +
"from Post p " +
"order by year(p.createdOn)", Integer.class)
.getResultList();
LOGGER.info("Publication years: {}", publicationYears);
应该将DISTINCT关键字传递给基础SQL语句，因为我们希望数据库引擎在返回结果集之前过滤重复项：
SELECT DISTINCT
extract(YEAR FROM p.created_on) AS col_0_0_
FROM
post p
ORDER BY
extract(YEAR FROM p.created_on)
-- Publication years: [2016, 2018]
对于实体查询，DISTINCT具有不同的含义。
不使用DISTINCT，查询如下：
List<Post> posts = entityManager
.createQuery(
"select p " +
"from Post p " +
"left join fetch p.comments " +
"where p.title = :title", Post.class)
.setParameter(
"title",
"High-Performance Java Persistence eBook has been released!"
)
.getResultList();
LOGGER.info(
"Fetched the following Post entity identifiers: {}",
posts.stream().map(Post::getId).collect(Collectors.toList())
);
将加入post和post_comment表，如下所示：
SELECT p.id AS id1_0_0_,
pc.id AS id1_1_1_,
p.created_on AS created_2_0_0_,
p.title AS title3_0_0_,
pc.post_id AS post_id3_1_1_,
pc.review AS review2_1_1_,
pc.post_id AS post_id3_1_0__
FROM post p
LEFT OUTER JOIN
post_comment pc ON p.id=pc.post_id
WHERE
p.title='High-Performance Java Persistence eBook has been released!'
-- Fetched the following Post entity identifiers: [1, 1]
但是父post记录在每个关联的post_comment行的结果集中重复。出于这个原因，List实体的Post将包含重复的Post实体引用。
要消除Post实体引用，我们需要使用DISTINCT：
List<Post> posts = entityManager
.createQuery(
"select distinct p " +
"from Post p " +
"left join fetch p.comments " +
"where p.title = :title", Post.class)
.setParameter(
"title",
"High-Performance Java Persistence eBook has been released!"
)
.getResultList();
LOGGER.info(
"Fetched the following Post entity identifiers: {}",
posts.stream().map(Post::getId).collect(Collectors.toList())
);
但是DISTINCT也被传递给SQL查询，这根本不可取：
SELECT DISTINCT
p.id AS id1_0_0_,
pc.id AS id1_1_1_,
p.created_on AS created_2_0_0_,
p.title AS title3_0_0_,
pc.post_id AS post_id3_1_1_,
pc.review AS review2_1_1_,
pc.post_id AS post_id3_1_0__
FROM post p
LEFT OUTER JOIN
post_comment pc ON p.id=pc.post_id
WHERE
p.title='High-Performance Java Persistence eBook has been released!'
-- Fetched the following Post entity identifiers: [1]
通过将DISTINCT传递给SQL查询，执行计划将执行额外的排序阶段，这会增加开销而不会带来任何值，因为父子组合总是返回唯一记录，因为子PK列：
Unique (cost=23.71..23.72 rows=1 width=1068) (actual time=0.131..0.132 rows=2 loops=1)
-> Sort (cost=23.71..23.71 rows=1 width=1068) (actual time=0.131..0.131 rows=2 loops=1)
Sort Key: p.id, pc.id, p.created_on, pc.post_id, pc.review
Sort Method: quicksort Memory: 25kB
-> Hash Right Join (cost=11.76..23.70 rows=1 width=1068) (actual time=0.054..0.058 rows=2 loops=1)
Hash Cond: (pc.post_id = p.id)
-> Seq Scan on post_comment pc (cost=0.00..11.40 rows=140 width=532) (actual time=0.010..0.010 rows=2 loops=1)
-> Hash (cost=11.75..11.75 rows=1 width=528) (actual time=0.027..0.027 rows=1 loops=1)
Buckets: 1024 Batches: 1 Memory Usage: 9kB
-> Seq Scan on post p (cost=0.00..11.75 rows=1 width=528) (actual time=0.017..0.018 rows=1 loops=1)
Filter: ((title)::text = 'High-Performance Java Persistence eBook has been released!'::text)
Rows Removed by Filter: 3
Planning time: 0.227 ms
Execution time: 0.179 ms
要从执行计划中消除排序阶段，我们需要使用HINT_PASS_DISTINCT_THROUGH JPA查询提示：
List<Post> posts = entityManager
.createQuery(
"select distinct p " +
"from Post p " +
"left join fetch p.comments " +
"where p.title = :title", Post.class)
.setParameter(
"title",
"High-Performance Java Persistence eBook has been released!"
)
.setHint(QueryHints.HINT_PASS_DISTINCT_THROUGH, false)
.getResultList();
LOGGER.info(
"Fetched the following Post entity identifiers: {}",
posts.stream().map(Post::getId).collect(Collectors.toList())
);
现在，SQL查询将不包含DISTINCT，但Post实体引用重复项将被删除：
SELECT
p.id AS id1_0_0_,
pc.id AS id1_1_1_,
p.created_on AS created_2_0_0_,
p.title AS title3_0_0_,
pc.post_id AS post_id3_1_1_,
pc.review AS review2_1_1_,
pc.post_id AS post_id3_1_0__
FROM post p
LEFT OUTER JOIN
post_comment pc ON p.id=pc.post_id
WHERE
p.title='High-Performance Java Persistence eBook has been released!'
-- Fetched the following Post entity identifiers: [1]
并且执行计划将确认我们这次不再有额外的排序阶段：
Hash Right Join (cost=11.76..23.70 rows=1 width=1068) (actual time=0.066..0.069 rows=2 loops=1)
Hash Cond: (pc.post_id = p.id)
-> Seq Scan on post_comment pc (cost=0.00..11.40 rows=140 width=532) (actual time=0.011..0.011 rows=2 loops=1)
-> Hash (cost=11.75..11.75 rows=1 width=528) (actual time=0.041..0.041 rows=1 loops=1)
Buckets: 1024 Batches: 1 Memory Usage: 9kB
-> Seq Scan on post p (cost=0.00..11.75 rows=1 width=528) (actual time=0.036..0.037 rows=1 loops=1)
Filter: ((title)::text = 'High-Performance Java Persistence eBook has been released!'::text)
Rows Removed by Filter: 3
Planning time: 1.184 ms
Execution time: 0.160 ms