mysql中具有相同表名的相同列名称

Question

tables

person_id   (primary key)

phs_people  (person_id,first_name,last_name)

phs_cutomers (person_id,company_name) 

phs_waiters (person_id,commission)

person_id is key between them.

So my question how can retrive customers firstname and last name, waiter firstname and lastname via person_id?

SELECT 
c.first_name AS customer_Fist_name,
c.last_name AS Customer_LastName, 
c.first_name AS WaiterFirstName, 
c.last_name AS Waiter_LastName,
invoice_number, amount_tendered, sale_time, DATE_FORMAT( sale_time, '%d-%m-%Y' ) AS sale_date, phs_sales.sale_id AS sale_id, SUM( item_unit_price * quantity_purchased * ( 1 - discount_percent /100 ) ) AS amount_due
FROM (
phs_sales
)
LEFT JOIN phs_people c ON c.person_id = phs_sales.customer_id
AND person_id = phs_sales.waiter_id
JOIN phs_sales_items ON phs_sales_items.sale_id = phs_sales.sale_id
LEFT JOIN (

SELECT sale_id, SUM( payment_amount ) AS amount_tendered
FROM phs_sales_payments
WHERE payment_type <> 'Check'
GROUP BY sale_id
) AS payments ON payments.sale_id = phs_sales.sale_id
GROUP BY sale_id
ORDER BY sale_time DESC
LIMIT 25

if I execute this query, I get the following error:

customer_Fist_name NULL,Customer_LastName NULL, WaiterFirstName NULL, Waiter_LastName NULL,

Answer 1

You want to do JOIN 's two times on the same table but with different values (customer's data and waiter's data), but you just use a JOIN once and give both conditions there.

To fix this, your have to JOIN the phs_people -Table twice like this:

...
LEFT JOIN phs_people AS c1 ON c1.person_id = phs_sales.customer_id
LEFT JOIN phs_people AS c2 ON c2.person_id = phs_sales.waiter_id
...

and then select the correct data like this:

SELECT
c1.first_name AS customer_Fist_name,
c1.last_name AS Customer_LastName,
c2.first_name AS WaiterFirstName,
c2.last_name AS Waiter_LastName,
...

PS: With this query, you should still get multiple NULL -Values, that's because half of your phs_sales -Table is filled with empty fields...