How to perform a left join in SQLALchemy?

Question

I have a SQL query which perfroms a series of left joins on a few tables:

SELECT
<some attributes>
FROM table1 t1
INNER JOIN table2 t2
      ON attr = 1 AND attr2 = 1
LEFT JOIN table3 t3
    ON t1.Code = t2.Code AND t3.Date_ = t1.Date_
LEFT JOIN tabl4 t4
    ON t4.Code = t1.code AND t4.Date_ = t1.Date_

So far, I have:

(sa.select([idc.c.Code])
            .select_from(
                t1.join(t2, and_(t1.c.attr == 1, t2.c.attr2 = 1))
                .join(t3, t3.c.Code == t1.c.Code)))

but I can’t figure out how to make the join a LEFT JOIN.

Asked By: user1742188

||

Source

Answer 1

The isouter=True flag will produce a LEFT OUTER JOIN which is the same as a LEFT JOIN.

With your code:

(sa.select([idc.c.Code])
        .select_from(
            t1.join(t2, and_(t1.c.attr == 1, t2.c.attr2 = 1))
            .join(t3, t3.c.Code == t1.c.Code, isouter=True)))

Declarative example:

session = scoped_session(sessionmaker())
session.query(Model).join(AnotherModel, AnotherModel.model_id == Model.id, isouter=True)

Answered By: user1742188

Answer 2

Here is how to use isouter:

select_from(db.join(Table1, Table2, isouter=True).join(Table3, isouter=True))

Answered By: melchoir55

Answer 3

Option 1 – LEFT JOIN and select ALL columns from both tables

# Query the db
results = db.session.query(Table_1, Table_2).join(
      Table_2, Table_2.column_name == Table_1.column_name, 
      isouter=True).all()
# Iterate results and do stuff
for result in results:
    try:
        # Use [0] for accesing table_1 columns (left table) and use [1] for accesing table_2 columns (right table)
        print(result[0].column_name_x)
        print(result[0].column_name_y)
        print(result[1].column_name_x)
        print(result[1].column_name_y)
     except Exception as e:
        print(str(e))

Option 2 – LEFT JOIN and select SOME columns from both tables

# Query the db
results = db.session.query(Table_1.column_name_x, Table_1.column_name_y Table_2.column_name_z).join(Table_2, Table_2.column_name == Table_1.column_name, isouter=True).all()
# Iterate results and do stuff
for result in results:
    try:
        # Use dot notation for accesing column from any table
        print(result.column_name_x)
        print(result.column_name_y)
        print(result.column_name_z)
    except Exception as e:
        print(str(e))

Answered By: IdoS

How to perform a left join in SQLALchemy?

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