SQL语句，当WHERE条件需要搜索工号为16或工号为2的雇员姓名时，需用到OR关键词如：

SELECT

 

[

EmployeeName

]

 

FROM

 

[

Employee

]

 

WHERE

 

[

WorkNumber

]

 

=

 

16

 

OR

 

[

WorkNumber

]

 

=

 

2

但是，上面SQL语句会涉及效率与性能问题，因为重写上面SQL语句：

SELECT

 

[

EmployeeName

]

 

FROM

 

[

Employee

]

 

WHERE

 

[

WorkNumber

]

 

=

 

16

UNION

 

ALL

SELECT

 

[

EmployeeName

]

 

FROM

 

[

Employee

]

 

WHERE

 

[

WorkNumber

]

 

=

 

2