break以及continue語句
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1. SELECT device_id,gender,age,university from user_profile where age!=''" 或者SELECT device_id,gender,age,university from user_profile where age<>''" 或者
SELECT device_id,gender,age,university from user_profile where age is not null 2. where university = "北京大學" or university = "復旦大學" or university = "山東大學" 等價於 where university in("北京大學" , "復旦大學" , "山東大學") 3. where university like '%北京%' 4. 查詢gpa最高的
select gpa
from user_profile
where university = '復旦大學'
order by gpa DESC
limit 1
或
select max(gpa)
from user_profile
where university = '復旦大學'
5. ROUND 函式用於把數值欄位舍入為指定的小數位數。SQL ROUND() 語法
SELECT
ROUND(column_name,decimals)
FROM
table_name
column_name 要舍入的欄位
decimals 要返回的小數位數
select count(gender) as male_num,round (avg(gpa) , 1) as avg_gpa
from user_profile
where gender = 'male'
6. 聚合函式結果作為篩選條件時,不能用where,而是用having語法
select university,
avg(question_cnt) as avg_question_cnt,
avg(answer_cnt) as avg_answer_cnt
from user_profile
group by university
having avg_question_cnt < 5 or avg_answer_cnt <20
7.
select device_id,question_id,result
from question_practice_detail
where device_id=(
select device_id
from user_profile
where university='浙江大學'
)
order by question_id;
或
select q.device_id,q.question_id,q.result
from question_practice_detail q join user_profile u on q.device_id=u.device_id(兩張表和在一起的條件)
where u.university='浙江大學'
order by q.question_id;
8. union all不用去重
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