问题
As we have seen that after you get a data source. We need to configure SQL dialects based on the database we use. After we select a particular dialect, How would that be used to make SQL queries specific to DB. Do frameworks like hibernate and JOOQ construct SQL queries in string based on the selected dialect ? If so which would be the most optimal way to support this in a framework of our own ?
回答1:
Do frameworks like hibernate and JOOQ construct SQL queries in string based on the selected dialect
Yes. In jOOQ, there's an internal StringBuilder that collects SQL fragments from your expression tree, which are generated for your target SQL dialect specifically. You can see how that works in action on this website: https://www.jooq.org/translate. Try translating for example this input: SELECT * FROM t LIMIT 1 (which could correspond to your jOOQ API usage ctx.selectFrom(T).limit(1). It translates to:
-- Oracle 12c and more
SELECT * FROM t FETCH NEXT 1 ROWS ONLY
-- Oracle 11g and less
SELECT *
FROM (
SELECT x.*, rownum rn
FROM (SELECT * FROM t) x
WHERE rownum <= 1
)
WHERE rn > 0
If so which would be the most optimal way to support this in a framework of our own ?
You need:
- An expression tree representation of your SQL query.
- Optionally, you can parse a string to build this expression tree, like jOOQ's parser if you want to support actual SQL, or you can have your own language abstraction like Hibernate did with HQL / JPQL
- Traverse that expression tree using something like a visitor to collect the SQL strings and bind variables.
But!
Do not build your own when you have off the shelf products like jOOQ or to some lesser extent Hibernate that can do the same. Building such a generic SQL abstraction is really difficult, and unless you want to actually sell such a product (you probably don't given your question), investing this time into building this product is not worth it at all.
The above LIMIT emulation is one of the more simple examples from jOOQ. Here's a lot more to help you decide against rolling your own, and that answer is still just scratching the surface of what jOOQ does behind the scenes.
来源：https://stackoverflow.com/questions/55909481/how-do-sql-dialects-actually-work-internally-in-frameworks-like-hibernate-and-jo