QueryDsl SQL - Left Join a subquery

Question

I am using QueryDsl SQL and I want to left join a subquery. Here is the query in plain SQL

SELECT
  usr.memberId,
  payoutsBbf.totalPyts
FROM
  users usr
  LEFT JOIN
    (SELECT
       pyt.member_id   AS mmb_id,
       SUM(pyt.amount) AS totalPyts
  FROM
    payout pyt
  WHERE
    pyt.payoutPeriod < '2018-01-01'
    GROUP BY pyt.member_id) AS payoutsBbf ON usr.id = payoutsBbf.mmb_id

I wish to write it in QueryDsl SQL and is almost important that the subquery is left joined to the main table users coz this is just a snippet of the whole complex query.

1. How do I deal with the LEFT JOIN x ON part

2. How do I deal with the subquery alias in the SELECT payoutBbf.totalPyts part

Answer 1

I think this will work for you. It's a bit hacky though:

SQLQueryFactory sqlqf; // Should be @Autowired

QUsers qusers = new QUsers();
QPayouts qpayouts = new QPayouts();

Expression<Long> memberId = ExpressionUtils.as(qpayouts.memberId, "mmb_id");
Expression<Double> totalPayouts = ExpressionUtils.as(qpayouts.amount.sum(), "totalPayouts");

SQLQuery<Tuple> payoutsBbf = SQLExpressions.select(memberId, totalPayouts)
            .from(qpayouts)                
            .where(qpayouts.payoutPeriod.lt("2018-01-01")) // Use date object
            .groupBy(qpayouts.memberId);

final SimplePath<? extends SQLQuery> payoutsBBfPath = Expressions.path(payoutsBBfPath.getClass(), "payoutsBbf");

List<Tuple> fetch = sqlqf.select(
        qusers.memberId,                
        Expressions.path(payoutsBbf.getClass(), new PathMetadata(payoutsBBfPath, "totalPayouts", PathType.PROPERTY))
    )
    .from(qusers)
    .leftJoin(payoutsBbf, payoutsBBfPath)
    .addJoinFlag(" on payoutsBbf.mmb_id = users.id", JoinFlag.Position.BEFORE_CONDITION)
    .fetch();

Note the usage of JoinFlag to specify the join column, using the alias defined as payoutsBbf . Also note the use of Expressions.path() to specify sub-columns in the select() section

Answer 2

this is how I would do it:

final StringPath payoutsBbf = stringPath("payoutsBbf");
final String mmbId = "mmb_id";
final String totalPyts = "totalPyts";
sqlQueryFactory.select(users.memberId, stringPath(payoutsBbf, totalPyts))
    .from(users).leftJoin(
        sqlQueryFactory.select(payout.member_id.as(mmbId), member_id.amount.sum().as(totalPyts))
            .from(payout).where(payout.payoutPeriod.lt("2018-01-01")).groupBy(payout.member_id),
        payoutsBbf
    ).on(users.id.eq(stringPath(payoutsBbf, mmbId))).fetch();