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Select..
LEFT JOIN Child c ON c.ParentAId = a.ParentAId
union
Select..
left Join Child c ON c.ParentBId = b.ParentBId
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SELECT columns
FROM table1
LEFT JOIN table2
ON table1.column_name = table2.column_name;
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SELECT Customers.customer_id, Customers.first_name, Orders.amount
FROM Customers
LEFT JOIN Orders
ON Customers.customer_id = Orders.customer;
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#LEFT JOIN: Return all rows from the left table, even if there are no matches in the right
#table
syntax->SELECT column_name(s)
FROM table_name1
LEFT JOIN table_name2
ON table_name1.column_name=table_name2.column_name
////example/////
SELECT Persons.LastName, Persons.FirstName, Orders.OrderNo
FROM Persons
LEFT JOIN Orders
ON Persons.P_Id=Orders.P_Id
ORDER BY Persons.LastName
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SELECT table1.column1, table2.column2
FROM table1
LEFT JOIN table2
ON table1.common_field = table2.common_field;