MySQL查询
-- 数据的准备
2 -- 创建一个数据库
3 create database python_test charset=utf8;
4
5 -- 使用一个数据库
6 use python_test;
7
8 -- 显示使用的当前数据是哪个?
9 select database();
10
11 -- 创建一个数据表
12 -- students表
13 create table students(
14 id int unsigned primary key auto_increment not null,
15 name varchar(20) default ‘‘,
16 age tinyint unsigned default 0,
17 height decimal(5,2),
18 gender enum(‘男‘,‘女‘,‘中性‘,‘保密‘) default ‘保密‘,
19 cls_id int unsigned default 0,
20 is_delete bit default 0
21 );
22
23 -- classes表
24 create table classes (
25 id int unsigned auto_increment primary key not null,
26 name varchar(30) not null
27 );
28
29
30
31 -- 查询
32 -- 查询所有字段
33 -- select * from 表名;
34 select * from students;
35 select * from classes;
36 select id, name from classes;
37
38 -- 查询指定字段
39 -- select 列1,列2,... from 表名;
40 select name, age from students;
41
42 -- 使用 as 给字段起别名
43 -- select 字段 as 名字.... from 表名;
44 select name as 姓名, age as 年龄 from students;
45
46 -- select 表名.字段 .... from 表名;
47 select students.name, students.age from students;
48
49
50 -- 可以通过 as 给表起别名
51 -- select 别名.字段 .... from 表名 as 别名;
52 select students.name, students.age from students;
53 select s.name, s.age from students as s;
54 -- 失败的select students.name, students.age from students as s;
55
56
57 -- 消除重复行
58 -- distinct 字段
59 select distinct gender from students;
60
61
62 -- 条件查询
63 -- 比较运算符
64 -- select .... from 表名 where .....
65 -- >
66 -- 查询大于18岁的信息
67 select * from students where age>18;
68 select id,name,gender from students where age>18;
69
70 -- <
71 -- 查询小于18岁的信息
72 select * from students where age<18;
73
74 -- >=
75 -- <=
76 -- 查询小于或者等于18岁的信息
77
78 -- =
79 -- 查询年龄为18岁的所有学生的名字
80 select * from students where age=18;
81
82
83 -- != 或者 <>
84
85
86 -- 逻辑运算符
87 -- and
88 -- 18到28之间的所以学生信息
89 select * from students where age>18 and age<28;
90 -- 失败select * from students where age>18 and <28;
91
92
93 -- 18岁以上的女性
94 select * from students where age>18 and gender="女";
95 select * from students where age>18 and gender=2;
96
97
98 -- or
99 -- 18以上或者身高查过180(包含)以上
100 select * from students where age>18 or height>=180;
101
102
103 -- not
104 -- 不在 18岁以上的女性 这个范围内的信息
105 -- select * from students where not age>18 and gender=2;
106 select * from students where not (age>18 and gender=2);
107
108 -- 年龄不是小于或者等于18 并且是女性
109 select * from students where (not age<=18) and gender=2;
110
111
112 -- 模糊查询
113 -- like
114 -- % 替换1个或者多个
115 -- _ 替换1个
116 -- 查询姓名中 以 "小" 开始的名字
117 select name from students where name="小";
118 select name from students where name like "小%";
119
120 -- 查询姓名中 有 "小" 所有的名字
121 select name from students where name like "%小%";
122
123 -- 查询有2个字的名字
124 select name from students where name like "__";
125
126 -- 查询有3个字的名字
127 select name from students where name like "__";
128
129 -- 查询至少有2个字的名字
130 select name from students where name like "__%";
131
132
133 -- rlike 正则
134 -- 查询以 周开始的姓名
135 select name from students where name rlike "^周.*";
136
137 -- 查询以 周开始、伦结尾的姓名
138 select name from students where name rlike "^周.*伦$";
139
140
141 -- 范围查询
142 -- in (1, 3, 8)表示在一个非连续的范围内
143 -- 查询 年龄为18、34的姓名
144 select name,age from students where age=18 or age=34;
145 select name,age from students where age=18 or age=34 or age=12;
146 select name,age from students where age in (12, 18, 34);
147
148
149
150 -- not in 不非连续的范围之内
151 -- 年龄不是 18、34岁之间的信息
152 select name,age from students where age not in (12, 18, 34);
153
154
155 -- between ... and ...表示在一个连续的范围内
156 -- 查询 年龄在18到34之间的的信息
157 select name, age from students where age between 18 and 34;
158
159
160 -- not between ... and ...表示不在一个连续的范围内
161 -- 查询 年龄不在在18到34之间的的信息
162 select * from students where age not between 18 and 34;
163 select * from students where not age between 18 and 34;
164 -- 失败的select * from students where age not (between 18 and 34);
165
166
167 -- 空判断
168 -- 判空is null
169 -- 查询身高为空的信息
170 select * from students where height is null;
171 select * from students where height is NULL;
172 select * from students where height is Null;
173
174 -- 判非空is not null
175 select * from students where height is not null;
176
177
178
179 -- 排序
180 -- order by 字段
181 -- asc从小到大排列，即升序
182 -- desc从大到小排序，即降序
183
184 -- 查询年龄在18到34岁之间的男性，按照年龄从小到到排序
185 select * from students where (age between 18 and 34) and gender=1;
186 select * from students where (age between 18 and 34) and gender=1 order by age;
187 select * from students where (age between 18 and 34) and gender=1 order by age asc;
188
189
190 -- 查询年龄在18到34岁之间的女性，身高从高到矮排序
191 select * from students where (age between 18 and 34) and gender=2 order by height desc;
192
193
194 -- order by 多个字段
195 -- 查询年龄在18到34岁之间的女性，身高从高到矮排序, 如果身高相同的情况下按照年龄从小到大排序
196 select * from students where (age between 18 and 34) and gender=2 order by height desc,id desc;
197
198
199 -- 查询年龄在18到34岁之间的女性，身高从高到矮排序, 如果身高相同的情况下按照年龄从小到大排序,
200 -- 如果年龄也相同那么按照id从大到小排序
201 select * from students where (age between 18 and 34) and gender=2 order by height desc,age asc,id desc;
202
203
204 -- 按照年龄从小到大、身高从高到矮的排序
205 select * from students order by age asc, height desc;
206
207
208 -- 聚合函数
209 -- 总数
210 -- count
211 -- 查询男性有多少人，女性有多少人
212 select * from students where gender=1;
213 select count(*) from students where gender=1;
214 select count(*) as 男性人数 from students where gender=1;
215 select count(*) as 女性人数 from students where gender=2;
216
217
218 -- 最大值
219 -- max
220 -- 查询最大的年龄
221 select age from students;
222 select max(age) from students;
223
224 -- 查询女性的最高 身高
225 select max(height) from students where gender=2;
226
227 -- 最小值
228 -- min
229
230
231 -- 求和
232 -- sum
233 -- 计算所有人的年龄总和
234 select sum(age) from students;
235
236
237 -- 平均值
238 -- avg
239 -- 计算平均年龄
240 select avg(age) from students;
241
242
243 -- 计算平均年龄 sum(age)/count(*)
244 select sum(age)/count(*) from students;
245
246
247 -- 四舍五入 round(123.23 , 1) 保留1位小数
248 -- 计算所有人的平均年龄，保留2位小数
249 select round(sum(age)/count(*), 2) from students;
250 select round(sum(age)/count(*), 3) from students;
251
252 -- 计算男性的平均身高 保留2位小数
253 select round(avg(height), 2) from students where gender=1;
254 -- select name, round(avg(height), 2) from students where gender=1;
255
256 -- 分组
257
258 -- group by
259 -- 按照性别分组,查询所有的性别
260 select name from students group by gender;
261 select * from students group by gender;
262 select gender from students group by gender;
263 -- 失败select * from students group by gender;
264
265 -- 计算每种性别中的人数
266 select gender,count(*) from students group by gender;
267
268
269 -- 计算男性的人数
270 select gender,count(*) from students where gender=1 group by gender;
271
272
273 -- group_concat(...)
274 -- 查询同种性别中的姓名
275 select gender,group_concat(name) from students where gender=1 group by gender;
276 select gender,group_concat(name, age, id) from students where gender=1 group by gender;
277 select gender,group_concat(name, "_", age, " ", id) from students where gender=1 group by gender;
278
279 -- having
280 -- 查询平均年龄超过30岁的性别，以及姓名 having avg(age) > 30
281 select gender, group_concat(name),avg(age) from students group by gender having avg(age)>30;
282
283 -- 查询每种性别中的人数多于2个的信息
284 select gender, group_concat(name) from students group by gender having count(*)>2;
285
286
287
288 -- 分页
289 -- limit start, count
290
291 -- 限制查询出来的数据个数
292 select * from students where gender=1 limit 2;
293
294 -- 查询前5个数据
295 select * from students limit 0, 5;
296
297 -- 查询id6-10（包含）的书序
298 select * from students limit 5, 5;
299
300
301 -- 每页显示2个，第1个页面
302 select * from students limit 0,2;
303
304 -- 每页显示2个，第2个页面
305 select * from students limit 2,2;
306
307 -- 每页显示2个，第3个页面
308 select * from students limit 4,2;
309
310 -- 每页显示2个，第4个页面
311 select * from students limit 6,2; -- -----> limit (第N页-1)*每个的个数, 每页的个数;
312
313 -- 每页显示2个，显示第6页的信息, 按照年龄从小到大排序
314 -- 失败select * from students limit 2*(6-1),2;
315 -- 失败select * from students limit 10,2 order by age asc;
316 select * from students order by age asc limit 10,2;
317
318 select * from students where gender=2 order by height desc limit 0,2;
319
320
321
322 -- 连接查询
323 -- inner join ... on
324
325 -- select ... from 表A inner join 表B;
326 select * from students inner join classes;
327
328 -- 查询 有能够对应班级的学生以及班级信息
329 select * from students inner join classes on students.cls_id=classes.id;
330
331 -- 按照要求显示姓名、班级
332 select students.*, classes.name from students inner join classes on students.cls_id=classes.id;
333 select students.name, classes.name from students inner join classes on students.cls_id=classes.id;
334
335 -- 给数据表起名字
336 select s.name, c.name from students as s inner join classes as c on s.cls_id=c.id;
337
338 -- 查询 有能够对应班级的学生以及班级信息，显示学生的所有信息，只显示班级名称
339 select s.*, c.name from students as s inner join classes as c on s.cls_id=c.id;
340
341 -- 在以上的查询中，将班级姓名显示在第1列
342 select c.name, s.* from students as s inner join classes as c on s.cls_id=c.id;
343
344 -- 查询 有能够对应班级的学生以及班级信息, 按照班级进行排序
345 -- select c.xxx s.xxx from student as s inner join clssses as c on .... order by ....;
346 select c.name, s.* from students as s inner join classes as c on s.cls_id=c.id order by c.name;
347
348 -- 当时同一个班级的时候，按照学生的id进行从小到大排序
349 select c.name, s.* from students as s inner join classes as c on s.cls_id=c.id order by c.name,s.id;
350
351 -- left join
352 -- 查询每位学生对应的班级信息
353