MySQL查询使用来自单独表的信息

Question

I have a rather complex SQL query that I am looking for a little help with.

I have two tables: a history table and a details table.

The history table contains the following columns.

Event Date(ev_date) 
Event Code(ev_code)  
Machine ID(mc_id)

The Details table contains the following columns:

Machine ID(mc_id), 
Location ID(lo_id) 
Machine Name(mc_name)

I need a query that returns the count of the number of events from the history table between a given date range of a given group of machines given by Location ID.

So, kinda in sudo code:

SELECT COUNT(*) 
FROM history 
WHERE ev_date (BETWEEN start_date AND end_date) AND ev_code = 1 AND ???? 

(mc_id must have certain lo_id from details table).

Does this make sense?

Thanks

Answer 1

SELECT COUNT(*)
    FROM history h
    WHERE h.ev_date BETWEEN @start_date AND @end_date
        AND ev_code = 1
        AND EXISTS(SELECT NULL
                       FROM details d
                       WHERE h.mc_id = d.mc_id
                           AND d.lo_id = @LocationID);

Answer 2

Assuming you have a one-to-one mapping between history and details on mc_id :

SELECT COUNT(*)
FROM history h
JOIN details d USING mc_id
WHERE h.ev_code = 1
AND h.ev_date between start_date and end_date
AND d.lo_id IN (?, ?, ?, ...)

Alternatively ON h.mc_id = d.mc_id instead of USING mc_id .