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How to register a function on Dialect from a function with parameters before and after the method?

I'm trying to make a JPQL run using the REGEXP function from MySQL 5.7 database on a H2 memory database. I would like to make a integration test using the same query.
As the REGEXP function does not exists on H2, I'm trying the register a new function to make it work, using the REGEXP_LIKE H2 function instead (just for the tests).
My query is:
String sql = "select o1.id from order o1 where :url REGEXP o1.regex";
But I can't figure out the right syntax to register the function. I'm trying something like this below, but I know that is incorrect anyway, because all examples that I found is mapping "normal" functions with parameters, but the REGEXP use a syntax like :url REGEXP o1.regex and not REGEXP(:url, o1.regex):
public class MyH2Dialect extends H2Dialect {
public MyH2Dialect() {
super();
registerFunction("REGEXP",
new SQLFunctionTemplate(StandardBasicTypes.BOOLEAN, "REGEXP_LIKE(?1, ?2, 'i')"));
}
}
And naturally the JPQL not recognize the function:
Caused by: org.hibernate.hql.internal.ast.QuerySyntaxException: unexpected token: REGEXP
I'm imaginig that I need to do something like this (pseudocode):
registerFunction("?1 REGEXP ?2",
new SQLFunctionTemplate(StandardBasicTypes.BOOLEAN, "REGEXP_LIKE(?1, ?2, 'i')"));
Any ideas?

I'm trying to make a JPQL run using the REGEXP function from MySQL 5.7 database on a H2 memory database.
Well, I couldn't register the function but I was able to run the REGEXP function on H2.
My mistake was trying to use JPQL instead of Native Query to make it run.
em.createNativeQuery(sql);
When I run the same query used on MySQL on the H2 database the SQL works as expected. So, I did not need to register the function. I couldn't understand that this was possible because the H2 documentation does not mention the REGEXP function but another question on SO mentioned that function exists.
Another possibility that I found (before make the above solution works) was upgrade the MySQL to 8.x version, because they create a new function called REGEXP_LIKE that have almost the same signature from the REGEXP_LIKE function from H2.

How to use IN clause with Mybatis Annotation inside SQL Provider

I have seen links pointing to the solution but most relevant is How to use Annotations with iBatis (myBatis) for an IN query?
but even this doesn't provide solution for Oracle driver.
public String getEmployees(Map<String, Object> params){
//Value hold by params params={empId={123,345,667,888}}
StringBuilder sql=new StringBuilder();
sql.append("Select * from employee where emp_id in (#{empId}");
Mybatis substitute the values from the params.
But when the value is substituted the query becomes some thing below.
Select * from employee where emp_id in ('123,345,667,888');
Which is a invalid Query as mybatis has added the single quotes in the query.
How should I handle this issue for a fix?
I cannot concatenate the values because to prevent SQL Injection.

The accepted answer in How to use Annotations with iBatis (myBatis) for an IN query? gives a solution working for postgres, a string representation of the list/array is passed and converted by the database.
Oracle does not support this. The list must be iterared to bind every value.
In my opinion, what you are looking for is Dynamic SQL, explained by LordOfThePigs in the next answer. Adapted to this case would be:
#Select({"<script>",
"SELECT *",
"FROM employee",
"WHERE emp_id IN",
"<foreach item='emp' collection='empId'",
"open='(' separator=', ' close=')'>",
"#{emp}",
"</foreach>",
"</script>"})
List<Employee selectEmployees(Map<String, Object> params);
#SelectProvider provides SQL string built in Java. But binding parameters become much more tedious.

JPA CriterialBuilder.concat force to use concat function

I'm using CriteriaBuilder.concat to concatenate 2 Strings, using code below:
Expression<String> concat = criteriaBuilder.concat(expr1, expr2)
But the generated SQL is something like:
select distinct col_1 || col_2
which causes org.hibernate.hql.ast.QuerySyntaxException:
expecting CLOSE, found '||' near line 1, column 48 [
select count(distinct generatedAlias0.hostname || generatedAlias0.device) from ...
^(1,48)
I wonder how to force it to generate the following SQL which uses the concat() function, instead of the || operator?
select distinct concat(col_1, col_2)
Update:
From the error we can see that the problem is more on the Hibernate (v3.6.10.Final) side, which is why making MySQL to accept || for concatenation doesn't help, also updating to a newer version is not an option for me.
Thank you

I've actually found a workaround. by using #Formula (from Hibernate) instead of CriteriaBuilder for the same task, like this:
#Entity
public class MyEntity {
#Column(name="col_a")
private String colA;
#Column(name="col_b")
private String colB;
#Formula("concat(col_a, col_b)")
private String concated;
//...
}
This way I can use the concated field for CriteriaBuilder.countDistinct:
//...
Expression<?> exp = criteriaBuilder.countDistinct(entity.get("concated"));
criteriaQuery.select(exp);
TypedQuery<Long> query = entityManager.createQuery(criteriaQuery);
return query.getSingleResult();
I wish JPA would (or hopefully already) support countDistinct with multiple columns, then all these mess could have been avoided (see: How to countDistinct on multiple columns, the answer was NO).

I had a similar problem with the concat function.
I have used the concat function in a selectCase and this also returns the same QuerySyntaxException.
My workaround is to use the concat function via criteria builder function:
cb().selectCase().when(cb().equal(root.get(Person_.flag), cb().literal("1")),
cb().function("CONCAT", String.class, root.get(Person_.something), cb().literal(" bla bla bla")))
.otherwise(root.get(Person_.something)))
Hibernate Version 4.3.11.Final

I had this issue as well. The JPA/HQL generated sql query use pipes as concat (which is ||).
I am using Mariadb 10, Springboot-data-jpa2.0.6 (with Hibernate 5.2.17)
Issue example
Given HQL: select x from Xxx x where concat(x.field1, x.field2) = $1
Generated SQL: select ..... where (x.field1 || x.field2) = ?
Reason:
Use of || is deprecated, since mysql 8.0, unless the PIPES_AS_CONCAT SQL mode is enabled: https://dev.mysql.com/doc/refman/8.0/en/mysql-nutshell.html
For mariadb: https://mariadb.com/kb/en/or/
Work around:
(preferred) ranther then use concat function, there is a similar one concat_ws: https://mariadb.com/kb/en/concat_ws/
Use JPA native query #Query(nativeQuery = true, value ="select * from ....")
set global sql_mode=<list of modes which contains PIPES_AS_CONCAT>
(I didn't try this)

JPA Criteria API using ISNULL in a order by statement

I use MySQL 5.5 with Hibernate 3.6 and JPA 2.0. I have a User table with a firstName which could also be null or an empty string. I want to have those empty firstName results last in my search results. For that I wrote the following SQL query which works just fine:
SELECT * FROM User ORDER BY ISNULL(firstName), firstName = "", firstName ASC LIMIT 100
Now want to translate this to JPA using the criteria API and I am not quite so sure about the order by. Here is what I have:
criteriaQuery = criteriaQuery.orderBy(cb.asc(cb.isNull(users.get(User_.firstName))), cb.asc(cb.equal(users.get(User_.firstName), "")), cb.asc(users.get(User_.firstName)));
However, the code snippet above does not work, because the CriteriaBuilder.isNull() method is traslated to IS NULL and not to the ISNULL() function of MySQL. I get the following exception:
org.hibernate.hql.ast.QuerySyntaxException: unexpected AST node: is null
Any ideas on how to check for null in the Order by statement with JPA 2.0

That is not possible. In JPA you can ORDER BY fields only that you select (that are in the SELECT part of your query). The problem is that there is no IS_NULL function, that can be used in the SELECT part.

I got the exact same problem as you do, finally I solve it using this way, maybe you can try:
CriteriaQuery<> query;
query.orderBy(cb.desc(cb.selectCase().
when(cb.isNull("field name"),0).otherwise(1)),
cb.asc("field name");

Referring to an earlier aliased field in a criteria query

In this query:
CriteriaBuilder cb = em.getCriteriaBuilder();
CriteriaQuery<Tuple> q = cb.createTupleQuery();
// FROM GamePlayedEvent gpe
Root<GamePlayedEvent> gpe = q.from(GamePlayedEvent.class);
// SELECT gameId, COUNT(*) AS count, AVG(duration)
// AS avDur, AVG(rewardCurrency) AS avCur, AVG(rewardXP) avXp
q.select(cb.tuple(
gpe.<String>get("gameId"),
cb.count(gpe).alias("count"),
cb.avg(gpe.<Double>get("duration")).alias("avDur"),
cb.avg(gpe.<Integer>get("rewardCurrency")).alias("avCur"),
cb.avg(gpe.<Integer>get("rewardXp")).alias("avXp")
));
// WHERE loginTime BETWEEN ...
q.where(cb.between(gpe.<Date>get("time"), fromTime, toTime));
// GROUP BY gameId
q.groupBy(gpe.<String>get("gameId"));
// ORDER BY count DESC
q.orderBy(cb.desc(???));
How can I add the ORDER BY count DESC, referring to the "count" defined in the SELECT clause?

What if you just captured the count expression, and used it directly?
Expression event_count = cb.count(gpe);
q.select(cb.tuple(
gpe.<String>get("gameId"),
event_count,
...
));
q.orderBy(cb.desc(event_count));

I came across the same problem today but none of the suggested solutions worked for me because I needed to reuse the expression not only in the order by clause but also in the group by clause.
One obvious solution would be to create a view on the database level but this is a bit clumsy, creates an unnecessary subquery and even not possible if the db user isn't granted enough privileges. A better option which I ended up implementing is to write something like this
q.select(cb.tuple(
gpe.<String>get("gameId"),
cb.count(gpe),
...
)).groupBy(cb.literal(2)).orderBy(cb.literal(2));
The first downside of this approach is that the code is errorprone. The other drawback is that the generated sql query contains ordinal position notation, which works on some databases (like postgresql or mariadb) but doesn't work on others (like sql server). In my case, however, I found this to be the best option.
Tested on jpa 2.1 with hibernate 5.2.3 as a provider and postgresql 9.6.

Even though the Pro JPA 2 book describes that the alias method can be used to generate a sql query alias (on page 251) I have had no success with making it work with neither EclipseLink or Hibernate. For your question I would say that your orderBy line should read:
q.orderBy(cb.desc(cb.count(gpe));
if it was supported by the different vendors.
As far as my research goes it seams that the alias method is only used for naming elements in the tuble used in the select (so only for projection).
I have one question though. Why would you want to use the JPA Criteria API for this query. It (the query) seams to be static in nature so why not use JPQL where you can define your query aliases directly.

Have you tried setting up a projection with an alias?
criteria.setProjection(Projections.projectionList()
.add(Projections.count("item.id"), "countItems"));
criteria.addOrder(Order.desc("countItems"));

For a sum aggregation field I have the following code which worked for me:
CriteriaBuilder cb = em.getCriteriaBuilder();
CriteriaQuery<T> cq = cb.createQuery(entity);
Root<T> root = cq.from(entity);
cq.orderBy(cb.desc(cb.sum(root.get(orderByString))));
// orderByString is string entity field that is being aggregated and which we want to put in orderby clause as well.