描述：

表 Variables:

+---------------+---------+
| Column Name   | Type    |
+---------------+---------+
| name          | varchar |
| value         | int     |
+---------------+---------+
在 SQL 中，name 是该表主键.
该表包含了存储的变量及其对应的值.

表 Expressions:

+---------------+---------+
| Column Name   | Type    |
+---------------+---------+
| left_operand  | varchar |
| operator      | enum    |
| right_operand | varchar |
+---------------+---------+
在 SQL 中，(left_operand, operator, right_operand) 是该表主键.
该表包含了需要计算的布尔表达式.
operator 是枚举类型, 取值于('<', '>', '=')
left_operand 和 right_operand 的值保证存在于 Variables 表单中.

计算表 Expressions 中的布尔表达式。

返回的结果表 无顺序要求 。

结果格式如下例所示。

示例 1：

输入：
Variables 表:
+------+-------+
| name | value |
+------+-------+
| x    | 66    |
| y    | 77    |
+------+-------+

Expressions 表:
+--------------+----------+---------------+
| left_operand | operator | right_operand |
+--------------+----------+---------------+
| x            | >        | y             |
| x            | <        | y             |
| x            | =        | y             |
| y            | >        | x             |
| y            | <        | x             |
| x            | =        | x             |
+--------------+----------+---------------+

输出:
+--------------+----------+---------------+-------+
| left_operand | operator | right_operand | value |
+--------------+----------+---------------+-------+
| x            | >        | y             | false |
| x            | <        | y             | true  |
| x            | =        | y             | false |
| y            | >        | x             | true  |
| y            | <        | x             | false |
| x            | =        | x             | true  |
+--------------+----------+---------------+-------+
解释：
如上所示, 你需要通过使用 Variables 表来找到 Expressions 表中的每一个布尔表达式的值.

数据准备：

Create Table If Not Exists Variables (name varchar(3), value int)
Create Table If Not Exists Expressions (left_operand varchar(3), operator ENUM('>', '<', '='), right_operand varchar(3))
Truncate table Variables
insert into Variables (name, value) values ('x', '66')
insert into Variables (name, value) values ('y', '77')
Truncate table Expressions
insert into Expressions (left_operand, operator, right_operand) values ('x', '>', 'y')
insert into Expressions (left_operand, operator, right_operand) values ('x', '<', 'y')
insert into Expressions (left_operand, operator, right_operand) values ('x', '=', 'y')
insert into Expressions (left_operand, operator, right_operand) values ('y', '>', 'x')
insert into Expressions (left_operand, operator, right_operand) values ('y', '<', 'x')
insert into Expressions (left_operand, operator, right_operand) values ('x', '=', 'x')

分析：

①观察两个表 不妨将x和y的值 都加到expressions表中

此时就需要使用两个左连接

select left_operand,v1.value as l,operator,right_operand ,v2.value as r
from Expressions left join Variables v1 on Expressions.left_operand = v1.name
                 left join Variables v2 on Expressions.right_operand = v2.name

②使用case when 根据操作符operator 列  判断 l 和 r 列值大小 并赋true 和false

select
    left_operand,
    operator,
    right_operand,
             case
             when operator = '>' then if(l > r,'true','false')
             when operator = '<' then if(l < r,'true','false')
             when operator = '=' then if(l = r,'true','false')
         end value
    from t1;

代码：

with t1 as (
select left_operand,v1.value as l,operator,right_operand ,v2.value as r
from Expressions left join Variables v1 on Expressions.left_operand = v1.name
                 left join Variables v2 on Expressions.right_operand = v2.name)
select
    left_operand,
    operator,
    right_operand,
             case
             when operator = '>' then if(l > r,'true','false')
             when operator = '<' then if(l < r,'true','false')
             when operator = '=' then if(l = r,'true','false')
         end value
    from t1;

总结：

① 一开始走了一些弯路 将两个值连接后 用concat 连接 判断值 后来发现都是true  

原因 是 concat连接的都当成字符串 无法计算

② case when  可以用两种解法 

解法一：

找出所有true 的条件 然后 else false

case when operator = '>' and l>r then 'true' 

 when operator = '<' and l<r then 'true' 

 when operator = '=' and l=r then 'true' 

else 'false' end

解法二：

另外一种就是上述代码中 then里面加个if判断  很巧妙 之前没用过