当前位置:网站首页>[turn to] MySQL operation practice (I): Keywords & functions
[turn to] MySQL operation practice (I): Keywords & functions
2022-07-05 05:14:00 【morpheusWB】
MySQL Practical operation ( One ): keyword & function
The following is the database sqltest Three tables in , Its structure and content ( part ) as follows :
1. keyword
①EXISTS、NOT EXISTS
EXISTS keyword : When EXISTS When the conditional statement in can return the record line , Condition is true , Returns the current loop This record I got ; conversely , At present loop The record we got was discarded .
NOT EXISTS keyword :NOT EXISTS And EXISTS contrary , That is to say EXISTS When the condition has a result set returned ,loop The records arrived will be discarded , Otherwise it would be loop The records to are added to the result set .
Count the students who did not take all the exams
mysql> SELECT * -> FROM t_stu_profile b -> WHERE NOT EXISTS( -> SELECT * -> FROM t_score a -> WHERE a.stu_id = b.stu_id) -> ;+--------+----------+--------+------+----------+| Stu_id | Stu_Name | Gender | Age | Class_id |+--------+----------+--------+------+----------+| 5 | Wang Wu | F | 17 | 0614 || 6 | Zhao Qi | F | 16 | 0615 |+--------+----------+--------+------+----------+
2. function
①COUNT()
COUNT() Functions can be used in two ways :
①COUNT(*): Count the number of rows in the table , Whether a table column contains null values (NULL) Or non null ;
②COUNT(col_name): Count rows with values in a specific column , Ignore NULL value ;
③COUNT(col_order): The effect same as above , When col_order=1, Then it means that the 1 Column to count . Application scenarios : Find the number of records of each course
# COUNT(*)mysql> SELECT lesson_id, COUNT(*) AS nums -> FROM t_score -> GROUP BY lesson_id -> ;+-----------+------+| lesson_id | nums |+-----------+------+| L001 | 4 || L002 | 4 || L004 | 1 || L003 | 3 || L005 | 2 |+-----------+------+# COUNT(col_name)mysql> SELECT lesson_id, COUNT(lesson_id) AS nums -> FROM t_score -> GROUP BY lesson_id -> ;# COUNT(col_order)mysql> SELECT lesson_id, COUNT(2) AS nums -> FROM t_score -> GROUP BY lesson_id -> ;
②GROUP_CONCAT()
GROUP_CONCAT() function : Realize grouping aggregation
GROUP_CONCAT(id ORDER BY id DESC SEPARATOR ‘_’)
③SUBSTRING_INDEX()
SUBSTRING_INDEX() function : segmentation
SUBSTRING_INDEX(str, delim, count)
str: Intercepted field
delim: Separator
count: Count ,count Being positive , Left to right , Take the first place n All of the contents to the left of the separator ;count Negative , From right to left , Take the first place n All the contents to the right of the separator
Application scenarios : Find the highest score of each subject
Implementation steps :
① Will table t_score Press lesson_id grouping ,GROUP_CONCAT(score ORDER BY score DESC SEPARATOR '_');
② utilize SUBSTRING_INDEX(t.scores, '_', 1) Function .
mysql> SELECT t.lesson_id, SUBSTRING_INDEX(t.scores, '_', 1) AS max_score -> FROM( -> SELECT lesson_id, GROUP_CONCAT(score ORDER BY score DESC SEPARATOR '_') AS scores -> FROM t_score -> GROUP BY lesson_id) t -> GROUP BY t.lesson_id -> ;+-----------+-----------+| lesson_id | max_score |+-----------+-----------+| L001 | 100 || L002 | 91 || L003 | 86 || L004 | 75 || L005 | 98 |+-----------+-----------+
————————————————
Copyright notice : This paper is about CSDN Blogger 「lulin916」 The original article of , follow CC 4.0 BY-SA Copyright agreement , For reprint, please attach the original source link and this statement .
Link to the original text :https://blog.csdn.net/weixin_39010770/article/details/86542676
边栏推荐
猜你喜欢
随机推荐
3dsmax scanning function point connection drawing connection line
UE 虚幻引擎,项目结构
PR first time
AutoCAD - set layer
PostgreSQL 超越 MySQL,“世界上最好的编程语言”薪水偏低
64 horses, 8 tracks, how many times does it take to find the fastest 4 horses at least
Do a small pressure test with JMeter tool
AutoCAD - scaling
中国AS树脂市场调研与投资预测报告(2022版)
Cocos2dx screen adaptation
2020-10-27
[转]: OSGI规范 深入浅出
嵌入式数据库开发编程(五)——DQL
2022/7/2做题总结
Pause and resume of cocos2dx Lua scenario
Research on the value of background repeat of background tiling
Common technologies of unity
Sqlserver stored procedures pass array parameters
UE4/UE5 虚幻引擎,材质篇(三),不同距离的材质优化
Embedded database development programming (VI) -- C API