加入两张桌子

Question

在此表1和表2中（AttendanceDate，EmployeeCode）是PRIMARY Key .. 我们如何用table2替换table1值，其中AttendanceDate和EmployeeCode将匹配..

与结果表一样..

Answer 1

尝试这个未经测试的查询：

select t1.AttendanceDate, t1.EmployeeCode, case when t2.duration is null then t1.duration else t2.duration end 
from table1 t1 left outer join table t2 on t1.AttendanceDate= t2.AttendanceDate and  t1.EmployeeCode =  t2.EmployeeCode

Answer 2

这应该有用。

SELECT Table1.AtendanceDate, Table1.EmployeeCode,
    CASE WHEN Table2.Duration IS NOT NULL THEN Table2.Duration ELSE Table1.Duration END AS Duration
FROM Table1 
LEFT OUTER JOIN Table2 ON Table1.AtendanceDate = Table2.AtendanceDate 
    AND Table1.EmployeeCOde = Table2.EmployeeCode