分组必统计，分组查询其实是排序
1 --使用in查询信息工程系和电子商务系的学生
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3 --查询信息工程系和电子商务系的学生
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5 select * from student where stuDept=\'信息工程系\' or stuDept=\'电子商务系\'
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7 select * from student where stuDept in(\'电子商务系\',\'信息工程系\')
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9
10 select * from student
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12 --使用count函数查询全体学生的人数
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14 select count(stuId) as 人数 from student
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16 select count(*) as 人数 from student
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18
19 /********************************/
20 --分组必统计
21 --使用group分组查询各系学生的数量
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23 --男生女生各多少人
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25
26
27 select * from student
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29 select stuSex, max(stuAvgrade) from student
30 group by stuSex
31
32 --查询男生和女生都有谁：(分组查询信息-都有谁-:是排序不是分组)
33 select stuSex, * from student
34 order by student.stuSex
35
36 --各系学生的数量
37 select stuDept,count(*) as 人数 from student
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group by stuDept
39
40 select * from student
41
42 计算机系 男 3
43 计算机系 男 1
44 电子商务系 男 1
45 电子商务系 女 1
46
47
48 --各系男生女生各多少人
49 select stuDept, stuSex,count(*) as 人数 from student
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group by stuDept,stuSex
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52
53 select stuDept, stuSex,count(*) as 人数 from student
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group by stuDept, stuSex
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56 --各系学生总分数
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58
59 select stuDept, sum(stuAvgrade) as 总成绩 from student
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group by stuDept
61
62 --每个系的成绩最好的
63 select stuDept, Max(stuAvgrade) as 最好的一个 from student
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group by stuDept
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66 select stuDept, Min(stuAvgrade) as 最差劲的一个 from student
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group by stuDept
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69
70 select stuDept, avg(stuAvgrade) as 平均 from student
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group by stuDept
72
73 select * from student
74
75 --统计各系的男生和女生各多少人
76 select stuDept,stuSex,COUNT(*) from student
77 group by stuDept, stuSex
78 order by stuDept --order by 排序
79
80
81 --查询各系学生信息
82
83
84
85 select * from student
86 group by stuDept,stuId,stuName
87
88 select stuDept, stuName, stuSex, stuBirth, stuSpeciality, stuAvgrade from student
89 group by stuDept, stuName, stuSex, stuBirth, stuSpeciality, stuAvgrade --这样写是可以的,其实组到最后，会发现等同于select * from student,也就是过分分组等于没有分组
90
91 --查询各系学生的信息，不是分组，因为分组必统计，这里其实是按系进行排序的概念
92 select student.stuDept, student.* from student
93 order by student.stuDept
94
95 --查询每个系的各专业的学生人数
96 select stuDept, stuSpeciality, count(*) from student
97 group by stuDept, stuSpeciality
98
99 --查询每个系的各专业的最好成绩
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101
102
103 select stuDept, stuSpeciality, max(stuAvgrade) from student
104 group by stuDept, stuSpeciality
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109 select stuDept, stuName, stuSex, stuBirth, stuSpeciality, stuAvgrade from student
110 order by stuDept -- order by 是排序关键字 dian, ji, xin
111
112 select * from student order by stuAvgrade Desc --desc是降序，默认值是Asc
113
114 select * from student order by stuAvgrade Asc
115
116
117 --使用having子句查询人数大于2的系
118
119 --查询人数大于2的系
120
121 --select stuDept from student where count(*) > 2
122
123
124 --//where子句是用于分组前的条件筛选//
125 select stuDept from student
126 where count(*) > 2
127 group by stuDept --非法，where条件部分不能有聚合函数
128
129 --select stuDept from student where count(*) > 2 group by stuDept 这样的写法是我们很自然就想到的，但是是非法，因为在Sql中不能在where条件后使用有计算的表达式，如聚合函数
130
131
132 --//having子句用于分组后的筛选
133 select stuDept, count(*) as 人数 from student
134 group by stuDept
135 having count(*) >= 2
136
137 select * from student
138
139
140
141
142 --查询人数大于1的系并且，不能是计算机系
143
144 --能在分组前的where子句中筛选的就一定要放在where子句中
145 select stuDept from student
146 group by stuDept
147 having count(*) >= 2 and stuDept <> \'计算机系\'
148
149 select stuDept from student
150 where stuDept <> \'计算机系\'
151 group by stuDept
152 having count(*) >= 2
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154
155
156
157 select stuDept as 系, count(*) as 人数, sum(stuAvgrade) as 总成绩, avg(stuAvgrade) as 平均成绩, max(stuAvgrade) as 最好成绩 from student
158 group by stuDept
159 having count(*) > 2
160
161 --查询平均成绩大于全体学生平均成绩的学生的信息
162
163
164 select * from student
165 where stuAvgrade > (
166
select AVG(stuAvgrade) from student
167 )