如何在jOOQ中的同一个表上写LEFT OUTER JOIN？

Question

how to write following SQL using jOOQ?

SELECT *
FROM food_db_schema.tblCategory AS t1
LEFT OUTER JOIN food_db_schema.tblCategory AS t2 ON t1.category_id = t2.parent_id
WHERE t2.parent_id IS NULL
AND t1.heartbeat = "ALIVE";

database is mySQL

Answer 1

flesk's answer depicts nicely how this can be done with jOOQ 1.x. A self-join using aliasing is more or less equivalent to a regular join using aliasing as described in the manual:

http://www.jooq.org/manual/DSL/ALIAS/

In the upcoming version 2.0, aliasing will be made less verbose and more type-safe. Hence flesk's solution could be simplified as such:

// Type-safe table aliasing:
TblCategory t1 = TBLCATEGORY.as("t1");
TblCategory t2 = TBLCATEGORY.as("t2");

Record record = create.select()
                      .from(t1)
                       // t1 and t2 give access to aliased fields:
                      .leftOuterJoin(t2).on(t1.CATEGORY_ID.equal(t2.PARENT_ID))
                      .where(t2.PARENT_ID.isNull())
                      .and(t1.HEARTBEAT.equal("ALIVE"));

I have also described a more complex example for a self-join here:

http://blog.jooq.org/2011/11/14/jooq-meta-a-hard-core-sql-proof-of-concept/

Answer 2

Maybe

SELECT *
FROM food_db_schema.tblCategory AS t1
WHERE t1.category_id IS NULL
AND t1.heartbeat = "ALIVE";

, but are you sure t2.parent_id is both supposed to be NULL and equal to t1.category_id ?

EDIT:

Then something like

Table<TblCategoryRecord> t1 = TBLCATEGORY.as("t1");
Table<TblCategoryRecord> t2 = TBLCATEGORY.as("t2");

Field<Integer> t1CategoryId = t1.getField(TblCategory.CATEGORY_ID);
Field<String> t1Heartbeat = t1.getField(TblCategory.HEARTBEAT);
Field<Integer> t2ParentId = t2.getField(TblCategory.PARENT_ID);

Record record = create.select().from(t1)
      .leftOuterJoin(t2).on(t1CategoryId.equal(t2ParentId))
      .where(t2ParentId.isNull())
      .and(t1Heartbeat.equal("ALIVE"));

depending on what the generated classes, properties and meta-model objects are called.