SQLAlchemy如何合并查询

Question

I am trying to create a database for a chess league I am trying to create.

I have written the query below which gives me the result I want but it looks very long-winded and I'd like a second opinion on how it could be written better.

I have two tables, one represents a team and another a "board". The board holds the result for each player and the board number they played.

class Board(db.Model):
    __tablename__ = 'board'
    id = db.Column(db.Integer, primary_key=True)
    board_number = db.Column(db.Integer)
    team_id = db.Column(db.Integer, db.ForeignKey("team.id"))
    result = db.Column(db.Float)

class Team(db.Model):
    __tablename__ = 'team'
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(50))
    boards = db.relationship('Board', backref='teams', lazy=True)

So to calculate the league table I have created 4 different queries: wins, losses, draws and total points.

They are then joined together and ordered by total points to create the league table.

    wins = (
    db.session.query(Team.id,
                     db.func.count(Board.result).label('win')
                     )
    .filter(Team.league_id == 1)
    .join(Board).filter_by(team_id=Team.id, result=1)
    .group_by(Team.id)
    .subquery()
)



losses = (
    db.session.query(Team.id,
                     db.func.count(Board.result).label('loss')
                     )
    .filter(Team.league_id == 1)
    .join(Board).filter_by(team_id=Team.id, result=0)
    .group_by(Team.id)
    .subquery()
)


draws = (
    db.session.query(Team.id,
                     db.func.count(Board.result).label('draw')
                     )
    .filter(Team.league_id == 1)
    .join(Board).filter_by(team_id=Team.id, result=0.5)
    .group_by(Team.id)
    .subquery()
)

total_points = (
    db.session.query(Team.id,
                     db.func.sum(Board.result).label('total')
                     )
    .filter(Team.league_id == 1)
    .join(Board, (Board.team_id == Team.id))
    .group_by(Team.id)
    .subquery()
)


league_table = (
    db.session.query(Team.name, wins.c.win, draws.c.draw, losses.c.loss, total_points.c.total)
    .join((wins, wins.c.id == Team.id))
    .join((losses, losses.c.id == Team.id))
    .join((draws, draws.c.id == Team.id))
    .join((total_points, total_points.c.id == Team.id))
    .order_by((total_points.c.total).desc())
    .all()
)

Could the 4 queries be merged into one?

Answer 1

The queries can be performed in a single query using the aggregate FILTER clause:

If FILTER is specified, then only the input rows for which the filter_clause evaluates to true are fed to the aggregate function;

In SQLAlchemy you can use FunctionElement.filter() to produce the clause:

db.session.query(
        Team.name,
        db.func.count(Board.result).filter(Board.result == 1).label('win'),
        db.func.count(Board.result).filter(Board.result == 0).label('loss'),
        db.func.count(Board.result).filter(Board.result == 0.5).label('draw'),
        db.func.sum(Board.result).label('total'))\
    .join(Board)\
    .filter(Team.league_id == 1)\
    .group_by(Team.id)\
    .order_by(db.func.sum(Board.result).desc())