Mysql 中select 語句單表查詢

1.檢視Mysql 內建函式

  1 # 檢視Mysql 內建函式
  2 mysql> help contents;
  3 You asked for help about help category: "Contents"
  4 For more information, type 'help <item>', where <item> is one of the following
  5 categories:
  6    Account Management
  7    Administration
  8    Compound Statements
 
  9    Data Definition
 10    Data Manipulation
 11    Data Types
 12    Functions
 13    Functions and Modifiers for Use with GROUP BY
 14    Geographic Features
 15    Help Metadata
 16    Language Structure
 17    Plugins
 18    Procedures
 19    Storage Engines
 20    Table Maintenance
 21    Transactions
 
 22    User-Defined Functions
 23    Utility
 24    
 25 # mysql> help Functions;
 26 You asked for help about help category: "Functions"
 27 For more information, type 'help <item>', where <item> is one of the following
 28 categories:
 29    Bit Functions
 30    Comparison operators
 
 31    Control flow functions
 32    Date and Time Functions
 33    Encryption Functions
 34    Information Functions
 35    Logical operators
 36    Miscellaneous Functions
 37    Numeric Functions
 38    String Functions
 39    
 40 # mysql> help String Functions     檢視字元功能函式
 41 You asked for help about help category: "String Functions"
 42 For more information, type 'help <item>', where <item> is one of the following
 43 topics:
 44    ASCII
 45    BIN
 46    BINARY OPERATOR
 47    BIT_LENGTH
 48    CAST
 49    CHAR FUNCTION
 50    CHARACTER_LENGTH
 51    CHAR_LENGTH
 52    CONCAT                                         #******
 53    CONCAT_WS
 54    CONVERT
 55    ELT
 56    EXPORT_SET
 57    EXTRACTVALUE
 58    FIELD
 59    FIND_IN_SET
 60    FORMAT
 61    FROM_BASE64
 62    HEX
 63    INSERT FUNCTION
 64    INSTR
 65    LCASE
 66    LEFT
 67    LENGTH
 68    LIKE
 69    LOAD_FILE
 70    LOCATE
 71    LOWER
 72    LPAD
 73    LTRIM
 74    MAKE_SET
 75    MATCH AGAINST
 76    MID
 77    NOT LIKE
 78    NOT REGEXP
 79    OCT
 80    OCTET_LENGTH
 81    ORD
 82    POSITION
 83    QUOTE
 84    REGEXP
 85    REPEAT FUNCTION
 86    REPLACE FUNCTION
 87    REVERSE
 88    RIGHT
 89    RPAD
 90    RTRIM
 91    SOUNDEX
 92    SOUNDS LIKE
 93    SPACE
 94    STRCMP
 95    SUBSTR
 96    SUBSTRING
 97    SUBSTRING_INDEX
 98    TO_BASE64
 99    TRIM
100    UCASE
101    UNHEX
102    UPDATEXML
103    UPPER
104    WEIGHT_STRING

2.select 單獨用法單獨使用

#1.select 配合內建函式使用
 #檢視當前使用者
 mysql> select user();
+----------------+
| user()         |
+----------------+
| root@localhost |
+----------------+
1 row in set (0.00 sec)
 #檢視當前mysql 版本
 mysql> select version();
+-----------+
| version() |
+-----------+
| 5.7.13    |
+-----------+
1 row in set (0.00 sec)
 #檢視當前時間
 mysql> select now();
+---------------------+
| now()               |
+---------------------+
| 2021-11-02 20:10:55 |
+---------------------+
1 row in set (0.00 sec)
#檢視當前在哪個資料庫
mysql> select database();
+------------+
| database() |
+------------+
| NULL       |
+------------+
1 row in set (0.00 sec)
#拼接命令   concat("")
mysql> select concat("hello world!");
+------------------------+
| concat("hello world!") |
+------------------------+
| hello world!           |
+------------------------+
1 row in set (0.01 sec)

mysql> select user,host from mysql.user;    #拼接格式為:mysql.sys@localhost
+-----------+-----------+
| user      | host      |
+-----------+-----------+
| mysql.sys | localhost |
| root      | localhost |
+-----------+-----------+
2 rows in set (0.00 sec)
mysql> select concat(user,"@",host) from mysql.user;   #拼接最終結果
+-----------------------+
| concat(user,"@",host) |
+-----------------------+
| mysql.sys@localhost   |
| root@localhost        |
+-----------------------+
2 rows in set (0.00 sec)

#2.計算
mysql> select 10 * 100;
+----------+
| 10 * 100 |
+----------+
|     1000 |
+----------+
1 row in set (0.00 sec)

mysql> select 100 / 10;
+----------+
| 100 / 10 |
+----------+
|  10.0000 |
+----------+
1 row in set (0.00 sec)

mysql> select 100 - 50;
+----------+
| 100 - 50 |
+----------+
|       50 |
+----------+
1 row in set (0.00 sec)

mysql> select 100 + 30;
+----------+
| 100 + 30 |
+----------+
|      130 |
+----------+
1 row in set (0.00 sec)

mysql> select 100 % 10;
+----------+
| 100 % 10 |
+----------+
|        0 |
+----------+
1 row in set (0.00 sec)

#3.查詢資料庫引數        
   #檢視全部引數: 504個
| transaction_prealloc_size                                | 4096                                   
| transaction_write_set_extraction                         | OFF                                   
| tx_isolation                                             | REPEATABLE-READ                       
| tx_read_only                                             | OFF                                   
| unique_checks                                            | ON                                     
| updatable_views_with_limit                               | YES                                   
| version                                                  | 5.7.13                                 
| version_comment                                          | Source distribution                   
| version_compile_machine                                  | x86_64                                 
| version_compile_os                                       | Linux                                 
| wait_timeout                                             | 28800                         
| warning_coun                                            | 0                                   
504 rows in set (0.00 sec) #版本不一樣,總的引數數量也不一樣

#檢視只記住引數部分時命令  用Like 
mysql> show variables like "%trx%";
+--------------------------------+-------+
| Variable_name                  | Value |
+--------------------------------+-------+
| innodb_api_trx_level           | 0     |
| innodb_flush_log_at_trx_commit | 1     |
+--------------------------------+-------+
2 rows in set (0.00 sec)


mysql> select @@datadir;           #檢視資料目錄
+-------------------+ 
| @@datadir               
+-------------------+
| /home/mysql/data/ |
+-------------------+
1 row in set (0.00 sec)

mysql> select @@socket;     #檢視socket 
+-----------------+
| @@socket        |        
+-----------------+
| /tmp/mysql.sock |
+-----------------+
1 row in set (0.00 sec)
mysql> select @@port;    #檢視port
+--------+
| @@port |                
+--------+
|   3306 |
+--------+
1 row in set (0.00 sec)

mysql> select @@innodb_flush_log_at_trx_commit;     
+----------------------------------+
| @@innodb_flush_log_at_trx_commit |
+----------------------------------+
|                                1 |
+----------------------------------+
1 row in set (0.00 sec)

3.select 標準用法 (配合其它子句使用)

#1.單表
預設執行順序(嚴格按照執行順序)
select * 
1. from      表1,表2,l...
2. where     過濾條件1 過濾條件2 ...
3. group by  條件列1 條件列2
4. select_list 
5. having    過濾條件1 過濾條件2 ...
6. order by  條件列1 條件列2
7. limit     限制條件

先匯入資料world.sql
[root@shell ~21:01:45]# mysql -uroot -p123456 <world.sql 
mysql: [Warning] Using a password on the command line interface can be insecure.

mysql> show databases;     #檢視所有庫
+--------------------+
| Database           |
+--------------------+
| information_schema |
| mysql              |
| performance_schema |
| sys                |
| world              |
+--------------------+
5 rows in set (0.00 sec)

mysql> use world;       #進入world庫
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A

Database changed
mysql> show tables;      #檢視world庫中所有表
+-----------------+
| Tables_in_world |
+-----------------+
| city            |
| country         |
| countrylanguage |
+-----------------+
3 rows in set (0.00 sec)

mysql> desc city;     #檢視city 表結構
+-------------+----------+------+-----+---------+----------------+
| Field       | Type     | Null | Key | Default | Extra          |
+-------------+----------+------+-----+---------+----------------+
| ID          | int(11)  | NO   | PRI | NULL    | auto_increment |
| Name        | char(35) | NO   |     |         |                |
| CountryCode | char(3)  | NO   | MUL |         |                |
| District    | char(20) | NO   |     |         |                |
| Population  | int(11)  | NO   |     | 0       |                |
+-------------+----------+------+-----+---------+----------------+
5 rows in set (0.00 sec)

mysql> show create table city;       #檢視city 建表語句
| Table | Create Table                                                                                 CREATE TABLE `city` (
  `ID` int(11) NOT NULL AUTO_INCREMENT,
  `Name` char(35) NOT NULL DEFAULT '',
  `CountryCode` char(3) NOT NULL DEFAULT '',
  `District` char(20) NOT NULL DEFAULT '',
  `Population` int(11) NOT NULL DEFAULT '0',
  PRIMARY KEY (`ID`),
  KEY `CountryCode` (`CountryCode`),
  CONSTRAINT `city_ibfk_1` FOREIGN KEY (`CountryCode`) REFERENCES `country` (`Code`)
) ENGINE=InnoDB AUTO_INCREMENT=4080 DEFAULT CHARSET=latin1 |

#1. select 配合from 子句使用
 語法:select 列 from 表; 類似 於cat /etc/passwd
  select * from 表 ;
---查詢表中所有資料
 select *  from world.city;
 select Id,name,countycode,district,population from world.city;
---查詢部分列值    類似於awk 取列
select name,population from world.city;

#2. select+from +where   其中where類似於grep 過濾
 #a.where 配合比較符來使用,例如: > = < >= <= !=
 查詢city表中,所有中國城市資訊
 mysql> select * from world.city where countrycode='CHN';
 查詢city表中,人口數小於1000的城市
 mysql> select * from world.city where population <1000;    
 #b.where 配合like 進行模糊查詢   適合於字串型別,不能like數字等
 查詢city表中,國家代號為CH開頭的城市資訊
 select * from world.city where countrycode like 'CH%';
 #注意:like語句在使用時,切記不可出現前面帶%的模糊查詢,因為不走索引
 #c.where 配合邏輯連結符  AND OR XOR(非)
 查詢中國人口數小於500萬的城市資訊
 select * from world.city where countrycode='CHN' AND population <5000000;
 查詢中國或者美國的城市資訊
 select * from world.city where countrycode='CHN' OR countrycode='USA';
 
 #3.select +from +where +group by   #分組後一定要顯示該列,
     select CountryCode, count(id/name)from world.city  group by CountryCode;
   #a.gtoup by 配合聚合函式  
   max()           #求最大值
   min()           #求最小值
   avg()           #求平均值
   count()         #統計個數
   sum()           #求和
   group_concat()  #列轉行
   說明:碰到group by  必然會有聚合函式
   統計每個國家的總的人口數
   select CountryCode, count(id/name)from world.city  group by CountryCode;
   統計中國,每個省的總人口數
   select District,sum(population) from city  where CountryCode='CHN' group by District;
   統計中國,每個省總人口,城市個數,城市名列表
   select District,sum(population),count(id) from city  where CountryCode='CHN' group by District;
 
#4.select +from +where +group by+having   #having 屬於後過濾,以group by 為中間,where 為前過濾.
  having 作用:與where 子句型別,having 屬於後過濾
  場景:需要在group by +聚合函式後,再做過濾時使用
   統計中國,每個省的總人口數並只顯示總人口數大於500萬
   select  District,sum(population)\
   from city\
   where CountryCode='CHN'\
   group by District\
   having sum(population) >5000000;

#5.select +from +where +group by+having +order by    
  order by 作用: 排序作用
  統計中國,每個省的總人口數,只顯示總人口數大於500萬並以總人口數以小到大進行排序
  select  District,sum(population)\
  from city\
  where CountryCode='CHN'\
  group by District\
  having sum(population) >5000000\
  order by sum(population);
  統計中國,每個省的總人口數,只顯示總人口數大於500萬並以總人口數以大到小進行排序
  select District,sum(population)\
  from city\
  where CountryCode='CHN'\
  group by District\
  having sum(population) >5000000\
  order by sum(population) desc;
  
#6.select +from +where +group by+having +order by+limit
  limit作用:分頁顯示結果集
  統計中國,每個省的總人口數,只顯示總人口數大於500萬並以總人口數以大到小進行排序,只顯示前5名
  select District,sum(population)\
  from city\
  where CountryCode='CHN'\
  group by District\
  having sum(population) >5000000\
  order by sum(population) desc\
  limit 5;
  
  統計中國,每個省的總人口數,只顯示總人口數大於500萬並以總人口數以大到小進行排序,只顯示6-10名
  select District,sum(population)\
  from city\
  where CountryCode='CHN'\
  group by District\
  having sum(population) >5000000\
  order by sum(population) desc\
  limit 5,5;  #意思為跳過5(左邊開始第2個5為跳),再顯示5,及為6-10名
  
  select District,sum(population)\
  from city\
  where CountryCode='CHN'\
  group by District\
  having sum(population) >5000000\
  order by sum(population) desc\
  limit 5 offset 5;  #意思為跳過5,再顯示5,及為6-10名
  
  顯示3-5行
  limit 2,3 或 limit 3 offset 2
  
  #7.select 的別名
  #a.列別名(在select 後面就是列),或者不接AS 直接別名也行.
  作用:
  1.做為好看
  2.group by having order by 子句呼叫,where 子句不行(因為執行順序原因)
  select District AS '城市名稱', sum(population) AS '總人口數'\
  from city\
  where CountryCode='CHN'\
  group by District\
  having sum(population) >5000000\
  order by sum(population) desc\
  limit 5 offset 5;
  
   #別名子名呼叫
  例子:
  select District AS '城市名稱', sum(population) AS '總人口數'\
  from city\
  where CountryCode='CHN'\
  group by 城市名稱\
  having 總人口數 >5000000\
  order by 總人口數 desc\
  limit 5 offset 5;
  
  #b.表別名
   作用:全域性呼叫定義表別名
   select a.sno AS '學號',a.sname AS '姓名',group_concat(c.cname) AS '課程名稱'\
   from student AS a\
   join sc AS b\
   on a.sno=b.sno\
   join course AS c\
   where a.sname='zhang3'\
   group by a.sno,a.sname;

Do everything well

Mysql 中select 語句單表查詢

Mysql 中select 語句單表查詢

mysql基礎--Select語句-多表查詢

5DQL之mysql單表查詢語句select

Mysql資料庫複習（1）----單表查詢sql語句

MySQL單表查詢操作例項詳解【語法、約束、分組、聚合、過濾、排序等】

MySQL單表查詢常見操作例項總結

Mysql中命令列操作表和庫的語句

MySQL筆記---單表查詢2

MySQL筆記---單表查詢1

MySQL使用聚合函式進行單表查詢

MySQL 單表查詢

Mysql之單表查詢

MySQL簡單查詢詳解-單表查詢

mysql 基礎入門單表查詢

MySQL -單表查詢

MySQL學習（6） - 單表查詢

Mysql 單表查詢-排序-分頁-group by初識

【資料庫MySQL】練習---簡單的單表查詢

MySQL資料庫 - 單表查詢（三）

【MySQL學習筆記（十一）】之基於成本的單表查詢優化計算與連線查詢的優化計算

Mysql 中select 語句單表查詢

相關推薦