MySQL案例数

Question

Hi I'm having a confusion on how to do this query.

I have tables:

transactiondetails
-> TransactionDetailsID
-> TerminalID
-> SiteID
-> TransactionType
-> Amount

terminals
-> TerminalID
-> TerminalName
-> TerminalType

Now, on table terminals.TerminalType the values is either 0 or 1. In my transactiondetails I need to find the total number of terminals.TerminalType = 1 in a particular SiteID in the table transactiondetails so my query is

SELECT 
CASE 
WHEN b.TerminalType IN (1) then COUNT(a.TerminalID)
ELSE 0 END AS TotalGenesis, a.SiteID
FROM npos.transactiondetails a
LEFT JOIN npos.terminals b ON a.TerminalID = b.TerminalID 
WHERE a.SiteID = 167;

But I got zero result for TotalGenesis even if I have a TerminalType = 1 in my transactiondetails.

Thanks for the help.

Answer 1

Use this query. Instead of using IN use equal(=) sign. See Below:

SELECT 
      CASE 
          WHEN b.TerminalType = 1 THEN COUNT(a.TerminalID)
          ELSE 0 
      END AS TotalGenesis, 
      a.SiteID
FROM npos.transactiondetails a
LEFT JOIN npos.terminals b ON a.TerminalID = b.TerminalID 
WHERE a.SiteID = 167;

Answer 2

Perhaps the join is failing? That would cause it to return zero. Are you sure TerminalType isn't null?

Do either of these queries return the correct result?

SELECT 
    CASE
        WHEN b.TerminalType IN (1) then COUNT(a.TerminalID)
        ELSE 0
    END AS TotalGenesis,
    a.SiteID
FROM npos.transactiondetails a
LEFT JOIN npos.terminals b ON a.TerminalID = b.TerminalID 
WHERE a.SiteID = 167
GROUP BY a.SiteID;

SELECT 
    SUM(CASE
        WHEN b.TerminalType IN (1) then 1
        ELSE 0
    END) AS TotalGenesis,
    a.SiteID
FROM npos.transactiondetails a
LEFT JOIN npos.terminals b ON a.TerminalID = b.TerminalID 
WHERE a.SiteID = 167;

Either way it appears that you want an inner join and not a left join.