MySQL 基础语句

#####一、直接量

select '直接量', host, user from mysql.user;

#####二、计算两个列的合

select 'Totle number: ', list1+ list2 from table_name;

#####三、打印显示列的别名

select list1 AS '别名' from table_name;

#####四、链接多个列

select CONCAT(list1, ' ', list2) AS '别名' from table_name;

#####五、表的别名

select * from table_name AS 别名;   #表的别名不须要加引号

#####六、查看带有空格的列

select `list name` from table_name;

##函数使用（字符函数） #####一、LEFT（显示列最左边的三个字符）

select LEFT(listtname, 3) from table_name;

#####二、RIGHT（显示列最右边的三个字符）

select RIGHT(listtname, 3) from table_name;

#####三、SUBSTRING(显示左边第三个字符到第八个字符的值)

select SUBSTRING(listname, 3, 8) from table_name;

#####四、LTRIM(去掉列左边的空格)

select LTRIM(listname) from table_name;

#####五、RTRIM(去掉列右边的空格)

select RTRIM(listname) from table_name;

#####六、UPPER(把全部小写转换为大写)

select UPPER(listname) from table_name;

#####七、LOWER(把全部大写转换为小写)

select LOWER(listname) from table_name;

#####八、CONCAT(链接多个列)

select CONCAT(list1, list2, list3, list4) from table_name;

##函数使用（复合函数）

select RIGHT(RTIRMR(listtname), 5) from table_name;

##函数使用(日期／时间函数) #####一、NOW

select NOW();

##函数使用（数值函数） #####一、ROUND（对任意数值进行四舍五入）

select ROUND(list_name, 0) from table_name;
#值为0时，表示没有小数位
#值为负数时，表示对小数点左侧前几位进行四舍五入
# 值为正数时，表示将数字四舍五入到指定的那么多个小数位

#####二、RAND(产生一个随机数)

select RAND();
select RAND(listname) from table_name;

#####三、PI（返回一个圆周率数值）

select PI();

#####四、CAST(转换函数)

select CAST(listname as DATETIME) AS '别名' from table_name;

#####五、IFNULL(把一个空值转换为一个有意义的值)

select IFNULL(listname, 'number') from table_name;

##排序数据 #####一、ORDER BY DESC

select list1, list2 from table_name ORDER BY list1;
select list1, list2 from table_name ORDER BY list1 DESC;    #降序排序
select * from table_name ORDER BY list1, list2;   #根据多个列进行排序
select CONCAT(list1, ' ', list2) as '别名' from table_name ORDER BY 别名;  #根据计算字段和别名进行排序

##基于列的逻辑 #####一、CASE WHEN THEN ELSE END (简单格式---对单一的列)

select CASE listtname WHEN 'V' THEN 'Version' WHEN 'S' THEN 'Some' ELSE 'Just so so' END as 'Bag' from table_name;

#####二、CASE WHEN THEN ELSE END (查询格式---对多个列)

select CASE WHEN list1='V' THEN 'Version' WHEN list2='S' THEN 'Some' ELSE 'Just so so' END as 'Bag' from table_name;

##基于行的逻辑 #####一、WHERE

select * from list_name where user = 'root';
select * from list_name where number > 5;    #number大于5
select * from list_name where number < 5;    #number 小于5
select * from list_name where number >= 5;  #number 大于等于5
select * from list_name where number <= 5;  #number 小于等于5
select * from list_name where number != 5;   #number 不等于5

#####二、LIMIT（限制输出行数）

select host, user from mysql.user limit 2;
select * from table_name ORDER BY list_name DESC LIMIT 5;  #对list_name进行倒序排序，只取5行数据

##布尔逻辑 一、AND OR NOT BETWEEN IN IS NULL

select * from table_name where state >= 5 AND state <=20;   #state的值大于等于5可是小于等于20
select * from table_name where state BETWEEN 5 AND 20;  #该语句同上面一条结果相同
select list1, list2 from table_name where state NOT BETWEEN state 5 AND 20;  #取反
select * from table_name where state = 'LI' OR state = 'YN';  #state值为LI，或者state值为YN
select * from table_name where state IN ('LI', 'YN');  #该语句同上面一条结果相同
select list1, list2 from table_name where state NOT IN ('LI', 'YN');  #取反
select list1, list2 from table_name where list_name IS NULL; #取值为空的行
select list1, list2 from table_name where list_name IS NOT NULL #取值不为空的行

##模糊匹配，使用表以下： ####一、LIKE

select * from SongTitles WHERE Artist LIKE '%Beat%' ;  #查找包含Beat的行
select * from SongTitles WHERE Artist LIKE '_he%'; #查找开头为任意字符，后面跟着he字符的行
# '%' 号为任意多个字符，'_'号为单个字符

##数据汇总, 使用表以下：Fees, Grades

####一、DISTINCT（消除重复）

select  DISTINCT Student from Grades;  #消除Student列的重复值
select DISTINCT CONCAT(Student, ' ', GradeType) AS 'Clear' from Grades; # 链接二个列，并消除重复的行

####二、SUM(计算合)

select SUM(Fee) AS 'TotalGym Fee: ' from Fees WHERE FeeType='Gym';  #计算全部学生的健身费用总合

####三、AVG（计算平均值）

select  AVG(Grade) from Grades WHERE GradeType='Quiz';  #计算Quiz的平均成绩

####四、MIN（计算最小值）

select MIN(Grade) from Grades WHERE GradeType='Quiz';  #计算Quiz的最小成绩

####五、MAX（计算最大值）

select MAX(Grade) from Grades WHERE GradeType='Quiz';  #计算Quiz的最大成绩

####六、AVG, MIN, MAX,SUM

select AVG(Grade) AS '平均值: ', MAX(Grade) AS '最大值: ', MIN(Grade) AS '最小值: ', SUM(Grade) AS '总合: ' from Grades WHERE GradeType='Quiz';   #计算Quiz的平均值，最大值，最小值，总合。

####七、COUNT (计算行数)

select COUNT(*) from Grades WHERE GradeType ="Homework";  #显示GradeType为Homework的全部行数
select COUNT(DISTINCT FeeType) from Fees;    #取消FeeType列重复的值，而后计算行数

####八、GROUP BY (分组数据)

select GradeType AS 'Group: ', MAX(Grade), MIN(Grade), SUM(Grade), AVG(Grade) from Grades GROUP BY GradeType;  #根据GradeType列分组，而且计算平均值，最大值，最小值，总合。

####九、聚合查询条件

select Student, GradeType, AVG(Grade) from Grades WHERE GradeType='Quiz' GROUP BY Student, GradeType HAVING AVG(Grade)>=71.5;  #根据Student，GradeType两个列进行分组而且计算出平均值，而后查询GradeType列为‘Quiz’的行，最后显示Grade列平均值大于71.5的行。