MySQL：在SELECT语句中使用IF条件比较日期

Question

Sample data:

db1.locationDetails table

| id | locationUID | locationName |
|----|-------------|--------------|
| 1  | L0001       | Site A       |
| 2  | L0002       | Site B       |
| 3  | L0003       | Site C       |
| 3  | L0004       | Site D       |

db2.HealthData table

| id | locationID  | Date_Time        | memUsage |
|----|-------------|------------------|----------|
| 1  | L0001       | 2018-09-10 11:43 |   35     |
| 2  | L0002       | 2018-09-10 08:22 |   39     |
| 3  | L0003       | 2018-09-10 14:44 |   43     |
| 4  | L0004       | 2018-09-10 16:01 |   72     |
| 5  | L0001       | 2018-09-12 01:26 |   50     |
| 6  | L0002       | 2018-09-12 03:15 |   32     |

I have a query:

SELECT DISTINCT db1.locationDetails.locationUID,
    db1.locationDetails.locationName,
    MAX(db2.HealthData.Date_Time),
    db2.HealthData.memUsage,
    IF(DATE(db2.HealthData.Date_Time) = '2018-09-12', "ON", "OFF") AS Status
FROM db1.locationDetails
LEFT JOIN db2.HealthData
ON db1.locationDetails.locationUID = db2.HealthData.locationID
GROUP BY db1.locationDetails.locationUID

Based on my understanding, the 'Status' column will show "ON" if the Date is equals to 2018-09-12 but somehow it always returns "OFF" regardless of whether the value in the Date_Time column is equal to the Date value specified in the query.

Can anyone tell me what is wrong here? Thanks in advance.

Expected output:

| locationUID | locationName | Date_Time       | memUsage | Status |
|-------------|--------------|-----------------|----------|--------|
| L0001       | Site A       |2018-09-12 01:26 |   50     | ON     |
| L0002       | Site B       |2018-09-12 03:15 |   32     | ON     |
| L0003       | Site C       |2018-09-10 14:44 |   43     | OFF    |
| L0004       | Site D       |2018-09-10 16:01 |   72     | OFF    |

Answer 1

Use subquery to get your desired result:

   select x.locationuid,x.locationname,maxitme, memusage, case when date(maxtime)='2018-09-12' then 'ON' else 'OFF' end as status
    from db1.locationDetails x
    inner join
    (select a.locationuid,maxtime,memusage
    from 
    (SELECT locationUID,MAX(Date_Time) as maxtime FROM db2.HealthData group by locationUID)a
    inner join db2.HealthData b on a.locationuid=b.locationuid)y
    on x.locationuid=y.locationuid

Answer 2

add Group by db1.locationDetails.locationUID,db2.HealthData.id

SELECT DISTINCT db1.locationDetails.locationUID,
        db1.locationDetails.locationName,
        MAX(db2.HealthData.Date_Time),
        db2.HealthData.memUsage,
        IF(DATE(db2.HealthData.Date_Time) = '2018-09-12', "ON", "OFF") AS Status
    FROM db1.locationDetails
    LEFT JOIN db2.HealthData
    ON db1.locationDetails.locationUID = db2.HealthData.locationID
    GROUP BY db1.locationDetails.locationUID,db2.HealthData.id