SQL：将表2的一列中的每个行单元格添加到表1中同一行的不同列单元格

Question

我有两个表（table1和table2），我希望得到最终结果

有谁知道如何在SQL语法中获得此结果？

表1语法     SELECT actID，日期     FROM table1     ORDER BY actID     ;

表2语法     SELECT动作，时间戳，时间     FROM table1     ORDER BY act，timestamp     ;

endResult语法： ??

Answer 1

最简单的方法是连接到表2四次，如下所示：

Select 
    t1.actID
    , t1.date
    , t25.time as t5
    , t27.time as t7
    , t28.time as t8
    , t210.time as t10
From table1 as t1
    left outer join table2 as t25
        on t25.act = t1.actid
        and t25.timestamp = 5
    left outer join table2 as t27
        on t27.act = t1.actid
        and t27.timestamp = 7
    left outer join table2 as t28
        on t28.act = t1.actid
        and t28.timestamp = 8
    left outer join table2 as t210
        on t210.act = t1.actid
        and t210.timestamp = 10

有意义吗？

Answer 2

我经常使用聚合来解决这个问题：

select t1.actid, t1.timestamp,
       max(case when t2.timestamp = 5 then t2.time end) as time_5,
       max(case when t2.timestamp = 7 then t2.time end) as time_7,
       max(case when t2.timestamp = 8 then t2.time end) as time_8,
       max(case when t2.timestamp = 10 then t2.time end) as time_10
from t1 left join
     t2
     on t1.actid = t2.act
group by t1.actid, t1.timestamp;