mysql连接成对相关的3张表

Question

► Context : I work in a museum (for real), people come everyday, they buy tickets for themselves (humans) and sometimes they also buy tickets for drinks and foods (objects). There are events, tickets have the same names per event but the prices are different.

► The problem : I have to create a report with 2 results : total sales (visitors + food + drinks) and how many people came (visitors only) for a specific event. Next is an image of the 3 tables in the database, how they relate and some sample data :

• Table TICKETS relates to SALES_MAIN through EVENT_ID column.
• Table SALES_MAIN relates to SALES_DETAIL through ID → MAIN_ID columns.
• Table SALES_DETAIL have a column TICKET_NAME but it's not unique in table TICKETS.

► The question : How to get both results, total sales and human count , for event 555 in one "select" ? I tried next 2 "select" but when I combine them with another INNER JOIN I get cartesian results :

Get detail sales for event 555 :

SELECT sales_detail.* FROM sales_main
INNER JOIN sales_detail ON sales_detail.main_id = sales_main.id
WHERE sales_main.event_id = '555'

Get tickets for event 555 :

SELECT * FROM tickets WHERE tickets.event_id = '555'

Answer 1

Use:

SELECT 
  SUM(CASE WHEN sd.ticket_name IN ('adult', 'child') THEN sd.quantity 
           ELSE 0 END) AS total_visitors,
  SUM(sd.quantity * t.price)  AS total_sales
FROM sales_main sm
JOIN sales_detail sd
  ON sd.main_id = sm.id
JOIN ticket t
  ON t.event_id = sm.event_id
 AND t.ticket_name = sd.ticket_name
WHERE sm.event_id = '555';

Conditional aggregation could also be based on type:

SUM(CASE WHEN t.ticket_type ='human' THEN sd.quantity ELSE 0 END)