Typesafe embedded DSLs like jOOQ are extremely powerful for dynamic SQL, because the query you’re constructing with the jOOQ DSL is a dynamic query by nature. You’re constructing a query expression tree using a convenient API (the “DSL”), even if you think your SQL statement is static. For instance:
The above query looks like a static SQL statement, the way you would write it in PL/SQL, for instance:
The PL/SQL implicit cursor loop runs over the records produced by a pre-compiled SQL statement. That’s not the case with the jOOQ statement, in case of which the Java runtime re-creates the jOOQ statement expression tree every time afresh by dynamically creating an org.jooq.Select object, step by step (more about how the DSL works here).
Using jOOQ for actual dynamic SQL
As we’ve seen before, all jOOQ statements are dynamic statements, even if they “feel” static. Sometimes, you actually want a dynamic SQL query, e.g. when the user is allowed to specify custom predicates. In this case, you could do something like this:
The above is not possible with PL/SQL easily, you’d have to resort to the dynamic SQL API called DBMS_SQL, which is about as verbose (and error-prone) as JDBC, as you’re concatenating SQL strings.
Adding functional programming to the mix
If you’re able to construct the entire query in a local scope, e.g. inside of a method, the above imperative style is quite sufficient. But sometimes, you may have something like a “base” query that you want to re-use all the time, and only sometimes, you want to add a custom predicate, or JOIN operation, etc.
In this case, using a more functional approach is optimal. For instance, you could offer a convenience API that produces a query fetching actor first and last names, with custom predicates:
The above utility method doesn’t actually execute the query, it just constructs it and takes a function as an argument. In the old days, this used to be called the “strategy pattern”, which is implemented much more easily with a function, than with an object oriented approach (see also Mario Fusco’s interesting blog series about the Gang of Four design patterns).
How to call the above utility? Easy!
Now, this is not versatile enough yet, as we can pass only one function. How about this, instead:
We can now call this with any number of input functions to form dynamic predicates. E.g.:
You get the idea.
Conclusion
… the idea is that jOOQ is an extremely powerful SQL expression tree API, which allows you to dynamically construct SQL queries of arbitrary complexity. If you’re running a static query, this just means that all of your SQL expressions are constant every time you execute the query.
There are no limits to how far you can push this. We’ve seen jOOQ users write queries that dynamically assemble dozens of common table expressions with several levels of dynamically nested derived tables, too. If you have a crazy example to share, we’re looking forward to it!
Filed under: jooq-in-use Tagged: dsl, dynamic SQL, embedded DSL, jooq, Query DSL, SQL expression tree, SQL generation, SQL generator, static SQL 
