#1440.计算布尔表达式的值

这题要计算表Expressions中的布尔值,因为Expressions表指记录了表达参数没有具体的值,不能使用数学计算符号,但是Variables记录着每个表达参数所对应的值,所以应该先将Expressions表与Variables表相连接,得到每个表答参数对应的值,即

select *

from expressions e

left join variables v1 on e.left_operand = v1.name

left join variables v2 on e.right_operand = v2.name

再使用case when建立一个列将数值比较结果与operator列对应符号相等就返回true,否则返回false这样就能将所有结果相列出,即

select left_operand,operator,right_operand,

case when v1.value > v2.value and operator = '>' then 'true'

when v1.value = v2.value and operator = '=' then 'true'

when v1.value < v2.value and operator = '<' then 'true'

else 'false'

end value

from expressions e

left join variables v1 on e.left_operand = v1.name

left join variables v2 on e.right_operand = v2.name