449 lines
16 KiB
Markdown
449 lines
16 KiB
Markdown
|
## SQL详解之DQL
|
|||
|
|
|
|||
|
|
接下来，我们利用之前创建的学校选课系统数据库，为大家讲解 DML 中的查询操作。无论对于开发人员还是数据分析师，查询都是非常重要的，它关系着我们能否从关系数据库中获取我们需要的数据。建议大家把上上一节课中建库建表的 DDL 以及 上一节课中插入数据的 DML 重新执行一次，确保表和数据跟没有问题再执行下面的操作。
|
|||
|
|
|
|||
|
|
```SQL
|
|||
|
|
USE school;
|
|||
|
|
|
|||
|
|
-- 查询所有学生的所有信息
|
|||
|
|
SELECT stu_id,
|
|||
|
|
stu_name,
|
|||
|
|
stu_sex,
|
|||
|
|
stu_birth,
|
|||
|
|
stu_addr,
|
|||
|
|
col_id
|
|||
|
|
FROM tb_student;
|
|||
|
|
|
|||
|
|
-- 查询学生的学号、姓名和籍贯(投影和别名)
|
|||
|
|
SELECT stu_id AS 学号,
|
|||
|
|
stu_name AS 姓名,
|
|||
|
|
stu_addr AS 籍贯
|
|||
|
|
FROM tb_student;
|
|||
|
|
|
|||
|
|
-- 查询所有课程的名称及学分(投影和别名)
|
|||
|
|
SELECT cou_name AS 课程名称,
|
|||
|
|
cou_credit AS 学分
|
|||
|
|
FROM tb_course;
|
|||
|
|
|
|||
|
|
-- 查询所有女学生的姓名和出生日期(数据筛选)
|
|||
|
|
SELECT stu_name,
|
|||
|
|
stu_birth
|
|||
|
|
FROM tb_student
|
|||
|
|
WHERE stu_sex = 0;
|
|||
|
|
|
|||
|
|
-- 查询籍贯为“四川成都”的女学生的姓名和出生日期(数据筛选)
|
|||
|
|
SELECT stu_name,
|
|||
|
|
stu_birth
|
|||
|
|
FROM tb_student
|
|||
|
|
WHERE stu_sex = 0
|
|||
|
|
AND stu_addr = '四川成都';
|
|||
|
|
|
|||
|
|
-- 查询籍贯为“四川成都”或者性别是女的学生(数据筛选)
|
|||
|
|
SELECT stu_name,
|
|||
|
|
stu_birth
|
|||
|
|
FROM tb_student
|
|||
|
|
WHERE stu_sex = 0
|
|||
|
|
OR stu_addr = '四川成都';
|
|||
|
|
|
|||
|
|
-- 查询所有80后学生的姓名、性别和出生日期(数据筛选)
|
|||
|
|
SELECT stu_name,
|
|||
|
|
stu_sex,
|
|||
|
|
stu_birth
|
|||
|
|
FROM tb_student
|
|||
|
|
WHERE '1980-1-1' <= stu_birth
|
|||
|
|
AND stu_birth <= '1989-12-31';
|
|||
|
|
|
|||
|
|
SELECT stu_name,
|
|||
|
|
stu_sex,
|
|||
|
|
stu_birth
|
|||
|
|
FROM tb_student
|
|||
|
|
WHERE stu_birth BETWEEN '1980-1-1' AND '1989-12-31';
|
|||
|
|
|
|||
|
|
-- 查询学分大于2的课程的名称和学分(数据筛选)
|
|||
|
|
SELECT cou_name,
|
|||
|
|
cou_credit
|
|||
|
|
FROM tb_course
|
|||
|
|
WHERE cou_credit > 2;
|
|||
|
|
|
|||
|
|
-- 查询学分是奇数的课程的名称和学分(数据筛选)
|
|||
|
|
SELECT cou_name,
|
|||
|
|
cou_credit
|
|||
|
|
FROM tb_course
|
|||
|
|
WHERE cou_credit MOD 2 <> 0;
|
|||
|
|
|
|||
|
|
-- 查询选择选了1111的课程考试成绩在90分以上的学生学号(数据筛选)
|
|||
|
|
SELECT stu_id
|
|||
|
|
FROM tb_record
|
|||
|
|
WHERE cou_id = 1111
|
|||
|
|
AND score > 90;
|
|||
|
|
|
|||
|
|
-- 查询名字叫“杨过”的学生的姓名和性别(数据筛选)
|
|||
|
|
SELECT stu_name AS 姓名,
|
|||
|
|
CASE stu_sex WHEN 1 THEN '男' ELSE '女' END AS 性别
|
|||
|
|
FROM tb_student
|
|||
|
|
WHERE stu_name = '杨过';
|
|||
|
|
|
|||
|
|
SELECT stu_name AS 姓名,
|
|||
|
|
IF(stu_sex, '男', '女') AS 性别
|
|||
|
|
FROM tb_student
|
|||
|
|
WHERE stu_name = '杨过';
|
|||
|
|
|
|||
|
|
-- 查询姓“杨”的学生姓名和性别(模糊匹配)
|
|||
|
|
-- 通配符 % 匹配零个或任意多个字符
|
|||
|
|
SELECT stu_name AS 姓名,
|
|||
|
|
CASE stu_sex WHEN 1 THEN '男' ELSE '女' END AS 性别
|
|||
|
|
FROM tb_student
|
|||
|
|
WHERE stu_name LIKE '杨%';
|
|||
|
|
|
|||
|
|
-- 查询姓“杨”名字两个字的学生姓名和性别(模糊匹配)
|
|||
|
|
-- 通过符 _ 匹配一个字符
|
|||
|
|
SELECT stu_name AS 姓名,
|
|||
|
|
CASE stu_sex WHEN 1 THEN '男' ELSE '女' END AS 性别
|
|||
|
|
FROM tb_student
|
|||
|
|
WHERE stu_name LIKE '杨_';
|
|||
|
|
|
|||
|
|
-- 查询姓“杨”名字三个字的学生姓名和性别(模糊匹配)
|
|||
|
|
SELECT stu_name AS 姓名,
|
|||
|
|
CASE stu_sex WHEN 1 THEN '男' ELSE '女' END AS 性别
|
|||
|
|
FROM tb_student
|
|||
|
|
WHERE stu_name LIKE '杨__';
|
|||
|
|
|
|||
|
|
-- 查询学号最后一位是3的学生的学号和姓名(模糊匹配)
|
|||
|
|
SELECT stu_id,
|
|||
|
|
stu_name
|
|||
|
|
FROM tb_student
|
|||
|
|
WHERE stu_id LIKE '%3';
|
|||
|
|
|
|||
|
|
-- 查询名字中有“不”字或“嫣”字的学生的学号和姓名(模糊匹配和并集运算)
|
|||
|
|
SELECT stu_id,
|
|||
|
|
stu_name
|
|||
|
|
FROM tb_student
|
|||
|
|
WHERE stu_name LIKE '%不%'
|
|||
|
|
OR stu_name LIKE '%嫣%';
|
|||
|
|
|
|||
|
|
SELECT stu_id,
|
|||
|
|
stu_name
|
|||
|
|
FROM tb_student
|
|||
|
|
WHERE stu_name LIKE '%不%'
|
|||
|
|
UNION
|
|||
|
|
SELECT stu_id,
|
|||
|
|
stu_name
|
|||
|
|
FROM tb_student
|
|||
|
|
WHERE stu_name LIKE '%嫣%';
|
|||
|
|
|
|||
|
|
-- 查询姓“杨”或姓“林”名字三个字的学生的学号和姓名(正则表达式模糊匹配)
|
|||
|
|
SELECT stu_id,
|
|||
|
|
stu_name
|
|||
|
|
FROM tb_student
|
|||
|
|
WHERE stu_name REGEXP '[林杨][\\u4e00-\\u9fa5]{2}';
|
|||
|
|
|
|||
|
|
-- 查询没有录入籍贯的学生姓名(空值处理)
|
|||
|
|
SELECT stu_name
|
|||
|
|
FROM tb_student
|
|||
|
|
WHERE TRIM(stu_addr) = ''
|
|||
|
|
OR stu_addr is null;
|
|||
|
|
|
|||
|
|
-- 查询录入了籍贯的学生姓名(空值处理)
|
|||
|
|
SELECT stu_name
|
|||
|
|
FROM tb_student
|
|||
|
|
WHERE TRIM(stu_addr) <> ''
|
|||
|
|
AND stu_addr is not null;
|
|||
|
|
|
|||
|
|
-- 查询学生选课的所有日期(去重)
|
|||
|
|
SELECT DISTINCT sel_date
|
|||
|
|
FROM tb_record;
|
|||
|
|
|
|||
|
|
-- 查询学生的籍贯(去重)
|
|||
|
|
SELECT DISTINCT stu_addr
|
|||
|
|
FROM tb_student
|
|||
|
|
WHERE TRIM(stu_addr) <> ''
|
|||
|
|
AND stu_addr is not null;
|
|||
|
|
|
|||
|
|
-- 查询男学生的姓名和生日按年龄从大到小排列(排序)
|
|||
|
|
SELECT stu_name,
|
|||
|
|
stu_birth
|
|||
|
|
FROM tb_student
|
|||
|
|
WHERE stu_sex = 1
|
|||
|
|
ORDER BY stu_birth ASC;
|
|||
|
|
|
|||
|
|
-- 补充：将上面的生日换算成年龄(日期函数、数值函数)
|
|||
|
|
SELECT stu_name AS 姓名,
|
|||
|
|
FLOOR(DATEDIFF(CURDATE(), stu_birth) / 365) AS 年龄
|
|||
|
|
FROM tb_student
|
|||
|
|
WHERE stu_sex = 1
|
|||
|
|
ORDER BY 年龄 DESC;
|
|||
|
|
|
|||
|
|
-- 查询年龄最大的学生的出生日期(聚合函数)
|
|||
|
|
SELECT MIN(stu_birth)
|
|||
|
|
FROM tb_student;
|
|||
|
|
|
|||
|
|
-- 查询年龄最小的学生的出生日期(聚合函数)
|
|||
|
|
SELECT MAX(stu_birth)
|
|||
|
|
FROM tb_student;
|
|||
|
|
|
|||
|
|
-- 查询编号为1111的课程考试成绩的最高分(聚合函数)
|
|||
|
|
SELECT MAX(score)
|
|||
|
|
FROM tb_record
|
|||
|
|
WHERE cou_id = 1111;
|
|||
|
|
|
|||
|
|
-- 查询学号为1001的学生考试成绩的最低分、最高分、平均分、标准差、方差(聚合函数)
|
|||
|
|
SELECT MIN(score) AS 最低分,
|
|||
|
|
MAX(score) AS 最高分,
|
|||
|
|
ROUND(AVG(score), 1) AS 平均分,
|
|||
|
|
STDDEV(score) AS 标准差,
|
|||
|
|
VARIANCE(score) AS 方差
|
|||
|
|
FROM tb_record
|
|||
|
|
WHERE stu_id = 1001;
|
|||
|
|
|
|||
|
|
-- 查询学号为1001的学生考试成绩的平均分，如果有null值，null值算0分(聚合函数)
|
|||
|
|
SELECT ROUND(SUM(score) / COUNT(*), 1) AS 平均分
|
|||
|
|
FROM tb_record
|
|||
|
|
WHERE stu_id = 1001;
|
|||
|
|
|
|||
|
|
-- 查询男女学生的人数(分组和聚合函数)
|
|||
|
|
SELECT CASE stu_sex WHEN 1 THEN '男' ELSE '女' END AS 性别,
|
|||
|
|
COUNT(*) AS 人数
|
|||
|
|
FROM tb_student
|
|||
|
|
GROUP BY stu_sex;
|
|||
|
|
|
|||
|
|
-- 查询每个学院学生人数(分组和聚合函数)
|
|||
|
|
SELECT col_id AS 学院编号,
|
|||
|
|
COUNT(*) AS 人数
|
|||
|
|
FROM tb_student
|
|||
|
|
GROUP BY col_id
|
|||
|
|
WITH ROLLUP;
|
|||
|
|
|
|||
|
|
-- 查询每个学院男女学生人数(分组和聚合函数)
|
|||
|
|
SELECT col_id AS 学院编号,
|
|||
|
|
CASE stu_sex WHEN 1 THEN '男' ELSE '女' END AS 性别,
|
|||
|
|
COUNT(*) AS 人数
|
|||
|
|
FROM tb_student
|
|||
|
|
GROUP BY col_id, stu_sex;
|
|||
|
|
|
|||
|
|
-- 查询每个学生的学号和平均成绩(分组和聚合函数)
|
|||
|
|
SELECT stu_id AS 学号,
|
|||
|
|
ROUND(AVG(score), 1) AS 平均分
|
|||
|
|
FROM tb_record
|
|||
|
|
GROUP BY stu_id;
|
|||
|
|
|
|||
|
|
-- 查询平均成绩大于等于90分的学生的学号和平均成绩(分组后的数据筛选)
|
|||
|
|
SELECT stu_id AS 学号,
|
|||
|
|
ROUND(AVG(score), 1) AS 平均分
|
|||
|
|
FROM tb_record
|
|||
|
|
GROUP BY stu_id
|
|||
|
|
HAVING 平均分 >= 90;
|
|||
|
|
|
|||
|
|
-- 查询1111、2222、3333三门课程平均成绩大于等于90分的学生的学号和平均成绩(分组前后的数据筛选)
|
|||
|
|
SELECT stu_id AS 学号,
|
|||
|
|
ROUND(AVG(score), 1) AS 平均分
|
|||
|
|
FROM tb_record
|
|||
|
|
WHERE cou_id in (1111, 2222, 3333)
|
|||
|
|
GROUP BY stu_id
|
|||
|
|
HAVING 平均分 >= 90
|
|||
|
|
ORDER BY 平均分 ASC;
|
|||
|
|
|
|||
|
|
-- 查询年龄最大的学生的姓名(子查询)
|
|||
|
|
SELECT stu_name
|
|||
|
|
FROM tb_student
|
|||
|
|
WHERE stu_birth = (SELECT MIN(stu_birth)
|
|||
|
|
FROM tb_student);
|
|||
|
|
|
|||
|
|
-- 查询选了两门以上的课程的学生姓名(子查询和集合运算)
|
|||
|
|
SELECT stu_name
|
|||
|
|
FROM tb_student
|
|||
|
|
WHERE stu_id in (SELECT stu_id
|
|||
|
|
FROM tb_record
|
|||
|
|
GROUP BY stu_id
|
|||
|
|
HAVING COUNT(*) > 2);
|
|||
|
|
|
|||
|
|
-- 查询学生的姓名、生日和所在学院名称(表连接)
|
|||
|
|
SELECT stu_name,
|
|||
|
|
stu_birth,
|
|||
|
|
col_name
|
|||
|
|
FROM tb_student AS t1, tb_college AS t2
|
|||
|
|
WHERE t1.col_id = t2.col_id;
|
|||
|
|
|
|||
|
|
SELECT stu_name,
|
|||
|
|
stu_birth,
|
|||
|
|
col_name
|
|||
|
|
FROM tb_student INNER JOIN tb_college
|
|||
|
|
ON tb_student.col_id = tb_college.col_id;
|
|||
|
|
|
|||
|
|
SELECT stu_name,
|
|||
|
|
stu_birth,
|
|||
|
|
col_name
|
|||
|
|
FROM tb_student NATURAL JOIN tb_college;
|
|||
|
|
|
|||
|
|
SELECT stu_name,
|
|||
|
|
stu_birth,
|
|||
|
|
col_name
|
|||
|
|
FROM tb_student CROSS JOIN tb_college;
|
|||
|
|
|
|||
|
|
-- 查询学生姓名、课程名称以及成绩(表连接)
|
|||
|
|
SELECT stu_name,
|
|||
|
|
cou_name,
|
|||
|
|
score
|
|||
|
|
FROM tb_student, tb_course, tb_record
|
|||
|
|
WHERE tb_student.stu_id = tb_record.stu_id
|
|||
|
|
AND tb_course.cou_id = tb_record.cou_id
|
|||
|
|
AND score is not null;
|
|||
|
|
|
|||
|
|
SELECT stu_name,
|
|||
|
|
cou_name,
|
|||
|
|
score
|
|||
|
|
FROM tb_student
|
|||
|
|
INNER JOIN tb_record
|
|||
|
|
ON tb_student.stu_id = tb_record.stu_id
|
|||
|
|
INNER JOIN tb_course
|
|||
|
|
ON tb_course.cou_id = tb_record.cou_id
|
|||
|
|
WHERE score is not null;
|
|||
|
|
|
|||
|
|
SELECT stu_name,
|
|||
|
|
cou_name,
|
|||
|
|
score
|
|||
|
|
FROM tb_student
|
|||
|
|
NATURAL JOIN tb_record
|
|||
|
|
NATURAL JOIN tb_course
|
|||
|
|
WHERE score is not null;
|
|||
|
|
|
|||
|
|
-- 补充：上面的查询结果取前5条数据(分页查询)
|
|||
|
|
SELECT stu_name,
|
|||
|
|
cou_name,
|
|||
|
|
score
|
|||
|
|
FROM tb_student
|
|||
|
|
NATURAL JOIN tb_record
|
|||
|
|
NATURAL JOIN tb_course
|
|||
|
|
WHERE score is not null
|
|||
|
|
ORDER BY cou_id ASC, score DESC
|
|||
|
|
LIMIT 5;
|
|||
|
|
|
|||
|
|
-- 补充：上面的查询结果取第6-10条数据(分页查询)
|
|||
|
|
SELECT stu_name,
|
|||
|
|
cou_name,
|
|||
|
|
score
|
|||
|
|
FROM tb_student
|
|||
|
|
NATURAL JOIN tb_record
|
|||
|
|
NATURAL JOIN tb_course
|
|||
|
|
WHERE score is not null
|
|||
|
|
ORDER BY cou_id ASC, score DESC
|
|||
|
|
LIMIT 5
|
|||
|
|
OFFSET 5;
|
|||
|
|
|
|||
|
|
-- 补充：上面的查询结果取第11-15条数据(分页查询)
|
|||
|
|
SELECT stu_name,
|
|||
|
|
cou_name,
|
|||
|
|
score
|
|||
|
|
FROM tb_student
|
|||
|
|
NATURAL JOIN tb_record
|
|||
|
|
NATURAL JOIN tb_course
|
|||
|
|
WHERE score is not null
|
|||
|
|
ORDER BY cou_id ASC, score DESC
|
|||
|
|
LIMIT 10, 5;
|
|||
|
|
|
|||
|
|
-- 查询选课学生的姓名和平均成绩(子查询和表连接)
|
|||
|
|
-- Error Code: 1248. Every derived table must have its own alias
|
|||
|
|
SELECT stu_name,
|
|||
|
|
avg_score
|
|||
|
|
FROM tb_student
|
|||
|
|
NATURAL JOIN (SELECT stu_id,
|
|||
|
|
ROUND(AVG(score), 1) AS avg_score
|
|||
|
|
FROM tb_record
|
|||
|
|
GROUP BY stu_id) as tmp;
|
|||
|
|
|
|||
|
|
-- 查询学生的姓名和选课的数量(子查询和表连接)
|
|||
|
|
SELECT stu_name,
|
|||
|
|
total
|
|||
|
|
FROM tb_student
|
|||
|
|
NATURAL JOIN (SELECT stu_id,
|
|||
|
|
COUNT(*) AS total
|
|||
|
|
FROM tb_record
|
|||
|
|
GROUP BY stu_id) as tmp;
|
|||
|
|
|
|||
|
|
-- 查询每个学生的姓名和选课数量(子查询和左外连接)
|
|||
|
|
SELECT stu_name AS 姓名,
|
|||
|
|
COALESCE(total, 0) AS 选课数量
|
|||
|
|
FROM tb_student AS t1
|
|||
|
|
LEFT JOIN (SELECT stu_id,
|
|||
|
|
COUNT(*) AS total
|
|||
|
|
FROM tb_record
|
|||
|
|
GROUP BY stu_id) AS t2
|
|||
|
|
ON t1.stu_id = t2.stu_id;
|
|||
|
|
```
|
|||
|
|
|
|||
|
|
有几个地方需要加以说明：
|
|||
|
|
|
|||
|
|
1. MySQL目前的版本不支持全外连接，上面我们通过`union`操作，将左外连接和右外连接的结果求并集实现全外连接的效果。大家可以通过下面的图来加深对连表操作的认识。
|
|||
|
|
|
|||
|
|
<img src="http://localhost/mypic/20211121135117.png" style="zoom:50%">
|
|||
|
|
|
|||
|
|
2. MySQL 中支持多种类型的运算符，包括：算术运算符（`+`、`-`、`*`、`/`、`%`）、比较运算符（`=`、`<>`、`<=>`、`<`、`<=`、`>`、`>=`、`BETWEEN...AND..`.、`IN`、`IS NULL`、`IS NOT NULL`、`LIKE`、`RLIKE`、`REGEXP`）、逻辑运算符（`NOT`、`AND`、`OR`、`XOR`）和位运算符（`&`、`|`、`^`、`~`、`>>`、`<<`），我们可以在 DML 中使用这些运算符处理数据。
|
|||
|
|
|
|||
|
|
3. 在查询数据时，可以在`SELECT`语句及其子句（如`WHERE`子句、`ORDER BY`子句、`HAVING`子句等）中使用函数，这些函数包括字符串函数、数值函数、时间日期函数、流程函数等，如下面的表格所示。
|
|||
|
|
|
|||
|
|
常用字符串函数。
|
|||
|
|
|
|||
|
|
| 函数 | 功能 |
|
|||
|
|
| --------------------------- | ----------------------------------------------------- |
|
|||
|
|
| `CONCAT` | 将多个字符串连接成一个字符串 |
|
|||
|
|
| `FORMAT` | 将数值格式化成字符串并指定保留几位小数 |
|
|||
|
|
| `FROM_BASE64` / `TO_BASE64` | BASE64解码/编码 |
|
|||
|
|
| `BIN` / `OCT` / `HEX` | 将数值转换成二进制/八进制/十六进制字符串 |
|
|||
|
|
| `LOCATE` | 在字符串中查找一个子串的位置 |
|
|||
|
|
| `LEFT` / `RIGHT` | 返回一个字符串左边/右边指定长度的字符 |
|
|||
|
|
| `LENGTH` / `CHAR_LENGTH` | 返回字符串的长度以字节/字符为单位 |
|
|||
|
|
| `LOWER` / `UPPER` | 返回字符串的小写/大写形式 |
|
|||
|
|
| `LPAD` / `RPAD` | 如果字符串的长度不足，在字符串左边/右边填充指定的字符 |
|
|||
|
|
| `LTRIM` / `RTRIM` | 去掉字符串前面/后面的空格 |
|
|||
|
|
| `ORD` / `CHAR` | 返回字符对应的编码/返回编码对应的字符 |
|
|||
|
|
| `STRCMP` | 比较字符串，返回-1、0、1分别表示小于、等于、大于 |
|
|||
|
|
| `SUBSTRING` | 返回字符串指定范围的子串 |
|
|||
|
|
|
|||
|
|
常用数值函数。
|
|||
|
|
|
|||
|
|
| 函数 | 功能 |
|
|||
|
|
| -------------------------------------------------------- | ---------------------------------- |
|
|||
|
|
| `ABS` | 返回一个数的绝度值 |
|
|||
|
|
| `CEILING` / `FLOOR` | 返回一个数上取整/下取整的结果 |
|
|||
|
|
| `CONV` | 将一个数从一种进制转换成另一种进制 |
|
|||
|
|
| `CRC32` | 计算循环冗余校验码 |
|
|||
|
|
| `EXP` / `LOG` / `LOG2` / `LOG10` | 计算指数/对数 |
|
|||
|
|
| `POW` | 求幂 |
|
|||
|
|
| `RAND` | 返回[0,1)范围的随机数 |
|
|||
|
|
| `ROUND` | 返回一个数四舍五入后的结果 |
|
|||
|
|
| `SQRT` | 返回一个数的平方根 |
|
|||
|
|
| `TRUNCATE` | 截断一个数到指定的精度 |
|
|||
|
|
| `SIN` / `COS` / `TAN` / `COT` / `ASIN` / `ACOS` / `ATAN` | 三角函数 |
|
|||
|
|
|
|||
|
|
常用时间日期函数。
|
|||
|
|
|
|||
|
|
| 函数 | 功能 |
|
|||
|
|
| ----------------------------------------- | ------------------------------------- |
|
|||
|
|
| `CURDATE` / `CURTIME` / `NOW` | 获取当前日期/时间/日期和时间 |
|
|||
|
|
| `ADDDATE` / `SUBDATE` | 将两个日期表达式相加/相减并返回结果 |
|
|||
|
|
| `DATE` / `TIME` | 从字符串中获取日期/时间 |
|
|||
|
|
| `YEAR` / `MONTH` / `DAY` | 从日期中获取年/月/日 |
|
|||
|
|
| `HOUR` / `MINUTE` / `SECOND` | 从时间中获取时/分/秒 |
|
|||
|
|
| `DATEDIFF` / `TIMEDIFF` / `TIMESTAMPDIFF` | 返回两个时间日期表达式相差多少天/小时 |
|
|||
|
|
| `MAKEDATE` / `MAKETIME` | 制造一个日期/时间 |
|
|||
|
|
|
|||
|
|
常用流程控制函数。
|
|||
|
|
|
|||
|
|
| 函数 | 功能 |
|
|||
|
|
| -------- | ------------------------------------------------ |
|
|||
|
|
| `IF` | 根据条件是否成立返回不同的值 |
|
|||
|
|
| `IFNULL` | 如果为NULL则返回指定的值否则就返回本身 |
|
|||
|
|
| `NULLIF` | 两个表达式相等就返回NULL否则返回第一个表达式的值 |
|
|||
|
|
|
|||
|
|
其他常用函数。
|
|||
|
|
|
|||
|
|
| 函数 | 功能 |
|
|||
|
|
| -------------------------- | ----------------------------- |
|
|||
|
|
| `MD5` / `SHA1` / `SHA2` | 返回字符串对应的哈希摘要 |
|
|||
|
|
| `CHARSET` / `COLLATION` | 返回字符集/校对规则 |
|
|||
|
|
| `USER` / `CURRENT_USER` | 返回当前用户 |
|
|||
|
|
| `DATABASE` | 返回当前数据库名 |
|
|||
|
|
| `VERSION` | 返回当前数据库版本 |
|
|||
|
|
| `FOUND_ROWS` / `ROW_COUNT` | 返回查询到的行数/受影响的行数 |
|
|||
|
|
| `LAST_INSERT_ID` | 返回最后一个自增主键的值 |
|
|||
|
|
| `UUID` / `UUID_SHORT` | 返回全局唯一标识符 |
|