jooq-获取单个值

Question

I had a question.. Why do I have repeat what I have selected in the fetch method.

Date minDate = getDSLContext()
    .select(CON_CAL_INSTANCE.DATE.min())
    .from(CON_CAL_INSTANCE)
    .join(CON_CAL_TEMPLATES)
    .on(CON_CAL_INSTANCE.TEMPLATE_ID.eq(CON_CAL_TEMPLATES.ID))
    .where(CON_CAL_TEMPLATES.ENTRY_TYPE.in("REPT", "HRPT"))
    .fetchOne(CON_CAL_INSTANCE.DATE.min());

So I have provided CON_CAL_INSTANCE.DATE.min() in my select clause, why do I have to repeat it in fetchOne(CON_CAL_INSTANCE.DATE.min()) ?

Or am I not doing this right?

Answer 1

You're doing it right. The way the jOOQ DSL is constructed with Java generics, your ResultQuery<Record1<Date>> doesn't "know" it is selecting only a single value, even if the ResultQuery uses Record1 as a row type.

Apart from repeating the column, you have some other options:

ResultQuery<Record1<Date>> query = // ...

// Use two method calls (this may result in a NullPointerException!
// as fetchOne() may return null):
Date date1 = query.fetchOne().value1();

// Use fetchValue():
Date date2 = getDSLContext().fetchValue(query);

See also the DSLContext.fetchValue() Javadoc .

Using a more LINQ-style syntax

On a side-note, there had been discussions in the past about using a more LINQ-style syntax in the jOOQ API:

from Table
where Predicates
select Projection

What may look like a good idea at first is raising new questions:

1. What about the ORDER BY , FOR UPDATE clauses, which should still be placed after SELECT . ( See this post for details ).
2. What about set operations, like UNION , INTERSECT and EXCEPT .

I've also written about the difference between lexical and logical order of operations in SQL, here

These open issues made us stick with the standard SQL syntax.