How to Perform a Left Join in Sqlalchemy?
I Have a Sql Query Which Perfroms a Series of Left Joins on a Few Tables: Select from Table1 T1 Inner Join Table2 T2 on Attr = 1 and Attr2 = 1 Left Join Table3...
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.
3 Answers
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)
Here is how to use isouter:
select_from(db.join(Table1, Table2, isouter=True).join(Table3, isouter=True))
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))
Elena Rostova holds a Master's degree in Public Health Journalism. She covers groundbreaking medical research, holistic wellness trends, mental health awareness, and nutritional science.
