Joins

Left Join Missing Customers

LEFT JOIN keeps every left row and fills missing right-side matches with NULL.

highlighted = computed this step

Orders input

Start with orders. Some customer keys are missing or NULL.

Orders\text{Orders}
Ordersorder_idcustomer_idamounto1c1120o2c280o3c_missing50o4NULL40

Customers input

Customers has only known customer keys.

Customers\text{Customers}
Customerscustomer_idsegmentc1retailc2wholesale

Join query

LEFT JOIN matches equal customer keys but keeps every order row.

Orders LEFT JOIN Customers\text{Orders LEFT JOIN Customers}
LEFT JOIN customersleft join customer_id=customer_idarity 5 rows 4Ordersarity 3 rows 4Customersarity 2 rows 2

Joined output

Matched orders get customer data; missing and NULL keys get NULL right-side cells.

left joined output\text{left joined output}
Joined outputorder_idcustomer_idamountCustomers.customer_idsegmento1c1120c1retailo2c280c2wholesaleo3c_missing50NULLNULLo4NULL40NULLNULL