转换数据类型

SQL 报表制作

Tyler Pernes

Learning & Development Consultant

本章目标

"脏"数据类型

问题 1：特定类型函数

问题 2：合并表（JOIN）

问题 2：合并表（UNION）

解读错误

特定类型函数错误：

SELECT AVG(first_name)
FROM athletes;

ERROR: Function avg(character varying) does not exist

JOIN 错误：

SELECT country, continent
FROM countries AS c1
JOIN continents AS c2
ON c1.continent_id = c2.id;

ERROR: Operator does not exist: integer = character varying

解决方案：用 CAST() 包起来

语法：

CAST(field AS type)

示例：

CAST(birthday AS date)
CAST(country_id AS int)

为函数进行 CAST

SELECT DATE_PART('month',birthdate)
FROM birthdates;

ERROR: Can't run DATE_PART on string.

SELECT DATE_PART('month', 
       CAST(birthdate AS date)) 
FROM birthdates;

+-----+
| 04  |
| 05  |
+-----+

为 JOIN 进行 CAST

SELECT a.id, b.id
FROM table_a AS a
JOIN table_b AS b
ON a.id = b.id;

ERROR: Cannot join ON varchar = int.

SELECT a.id, b.id
FROM table_a AS a
JOIN table_b AS b
ON a.id = CAST(b.id AS varchar);

Query Ran Successfully!

规划数据类型问题

出现就修复
仔细读错误信息！

规划数据类型问题

SELECT column_name, data_type
FROM information_schema.columns
WHERE table_name = 'countries';

+--------------+--------------------+
| column_name  | data_type          |
|--------------|--------------------|
| id           | integer            |
| country      | character varying  |
| region       | character varying  |
+--------------+--------------------+

数据类型文档

https://www.postgresql.org/docs/9.5/datatype.html

练习时间！

SQL 报表制作