CAST(expr AS )

• 定义：cast(expr as <type>)
• 说明：将计算结果expr转换为type类型，如果转换失败，返回NULL，将一个非空字符串转换为BOOLEAN类型，会返回true，0转换为BOOLEAN是false，非0转换为BOOLEAN是true
• 返回值类型：type
• 案例

SELECT CAST('1' AS INT);
+------+
| _c0  |
+------+
| 1    |
+------+

SELECT CAST('1' AS BOOLEAN);
+-------+
|  _c0  |
+-------+
| true  |
+-------+

SELECT CAST('' AS BOOLEAN);
+--------+
|  _c0   |
+--------+
| false  |
+--------+

SELECT CAST(0 AS BOOLEAN);
+--------+
|  _c0   |
+--------+
| false  |
+--------+

SELECT CAST(-1 AS BOOLEAN);
+-------+
|  _c0  |
+-------+
| true  |
+-------+

SELECT CAST('Hello' AS INT);
+-------+
|  _c0  |
+-------+
| NULL  |
+-------+