mysql多个连接同一个表

Question

我有一张表格，列有日期、系统、硬度、电导率、tankno。

日期系统 cond hard tankno 2014-01-04 HT 55.67 2.68 1 2014-01-05 HT 64.67 3.1 2 2014-01-10 HT 54.77 2.44 1 2014-01-10 HT 66.3 3.47 2 2014-01-18 HT 55.63 2.38 1 2014-01-18 HT 65.9 1.44 2 2014-01-24 HT 53.1 1.76 2 2014-01-28 HT 64.53 2.1 1

我想要这样的报告

日期系统 Tank 1 cond Tank1 Hard Tank 2 cond Tank 2hard 2014-01-04 HT 55.67 2.68 NULL NULL 2014-01-05 HT NULL NULL 64.67 3.1 2014-01-10 HT 54.77 2.44 66.3 3.47 2014-01-18 HT 55.63 2.38 65.9 1.44 2014-01-24 HT NULL NULL 53.1 1.76 2014-01-28 HT 64.53 2.1 NULL NULL

我正在使用这个 sql：

select t1.date, t1.cond as tk1cond, t1.hard as tk1hard,
       t2.cond as tk2cond, t2.hard as tk2hard   
from systemvalue t1 left join systemavalue t2 
     on t1.date=t2.date 
     and t2.tankno='2' 
     and  t2.system='HT' 
     and month(t2.date)='01' 
     and year(t2.date)='2014' 
where t1.tankno='1' and t1.system='HT'
      and month(t1.date)='01' 
      and year(t1.date)='2014'

但此返回数据与 tankno 1 值可用的日期有关，如下所示：

日期系统 Tank 1 cond Tank1 Hard Tank 2 cond Tank 2hard 2014-01-04 HT 55.67 2.68 NULL NULL 2014-01-10 HT 54.77 2.44 66.3 3.47 2014-01-18 HT 55.63 2.38 65.9 1.44 2014-01-28 HT 64.53 2.1 NULL NULL

如何修改sql以获取所有日期的数据

我想要日期列中的所有日期，并将其连接到所有 tank 1 vlaues 和 tank 2 值

Answer 1

你在这里：

select 
    dt.date, 
    t1.cond as tk1cond, 
    t1.hard as tk1hard,
    t2.cond as tk2cond, 
    t2.hard as tk2hard  
from (select date from systemvalue group by date) dt
left join systemvalue t1 on t1.date = dt.date and t1.tankno = '1' and t1.system = 'HT'
left join systemvalue t2 on t2.date = dt.date and t2.tankno = '2' and t2.system = 'HT'