SQL advanced statement select usage
Create a sample table
[root@mysql1 ~]# mysql -uroot -p
Enter password:
.....此处省略
mysql> create database school;
Query OK, 1 row affected (0.00 sec)
mysql> use school;
Database changed
##创建成绩表##
mysql> create table result(id int(3), name varchar(64), Maths int(3), English int(3));
Query OK, 0 rows affected (0.01 sec)
mysql> insert into result values(001,'zhangsan',90,85),(002,'lisi',90,80),(003,'wangwu',87,85),(004,'zhaoliu',93,80),(005,'xiaoming',80,95),(006,'xiaohong',95,87),(007,'xiaogang',83,89);
Query OK, 7 rows affected (0.00 sec)
Records: 7 Duplicates: 0 Warnings: 0
mysql> select * from result;
+------+----------+-------+---------+
| id | name | Maths | English |
+------+----------+-------+---------+
| 1 | zhangsan | 90 | 85 |
| 2 | lisi | 90 | 80 |
| 3 | wangwu | 87 | 85 |
| 4 | zhaoliu | 93 | 80 |
| 5 | xiaoming | 80 | 95 |
| 6 | xiaohong | 95 | 87 |
| 7 | xiaogang | 83 | 89 |
+------+----------+-------+---------+
7 rows in set (0.01 sec)
##创建宿舍表
mysql> create table dormitory(id int(3), room int(3));
Query OK, 0 rows affected (0.01 sec)
mysql> insert into dormitory values(001,501),(002,501),(003,502),(004,502),(005,502),(006,503),(007,503);
Query OK, 7 rows affected (0.00 sec)
Records: 7 Duplicates: 0 Warnings: 0
mysql> select * from dormitory;
+------+------+
| id | room |
+------+------+
| 1 | 501 |
| 2 | 501 |
| 3 | 502 |
| 4 | 502 |
| 5 | 502 |
| 6 | 503 |
| 7 | 503 |
+------+------+
7 rows in set (0.00 sec)
mysql>
Keyword ranking
Use the ORDER BY statement to achieve sorting.
Sorting can be based on one or more fields
grammar structure
SELECT colum1,colum2,....FROM table_name ORDER BY colum1,colum2,.... ASC|DESC;
ASC：Ascending order, the default sorting method
DESC: Descending order
Sort by single field
Sort by English score in descending order
select id,name,Maths,English from result order by English desc;
Sort in ascending order of math grades
select id,name,Maths,English from result order by Maths;
Sort by multiple fields
Sort in ascending order of math scores, same as math scores, sorted in ascending order of English scores
select id,name,Maths,English from result order by Maths,English;
Group the results
Use the group by statement to achieve grouping,
usually combined with aggregate functions. The
results can be grouped by one or more fields.
Group those with math scores over 85 and count the same number of math scores
select count(name),Maths from result where Maths>85 group by Maths;
Sort the above results in descending order
select count(name),Maths from result where Maths>85 group by Maths order by Maths desc;
Limit result entries
Only return the first row or the first few rows of the select query result
Use the limit statement to limit entries
limit syntax structure
select 字段1，字段2 from 表名 limit [offset,] number；
offset: Position offset, starting from 0
number: Return the maximum number of record lines
Only show the default first three rows of the score table
select * from result limit 3;
Display three lines after the third line
select * from result limit 2,3;
Only display the top three IDs, names, and English scores of the top three English scores
select id,name,English from result order by English desc limit 3;
Set alias
Use the as statement to set the alias, the keyword as can be omitted. When
setting the alias, ensure that it does not conflict with the names of other tables or fields in the library
Syntax structure of
alias Field alias: select 字段 as 别名 from 表名；
table alias:select 字段 from 表名 as 别名
Set the alias for the id in the result table as the student number
select id as xuehao from result;
Two tables connected query
Both result and dormitory have a common column id, which connects the two tables through the same field
select * from result inner join dormitory on result.id=dormitory.id;
Query name and corresponding dormitory
select result.name,dormitory.room from result inner join dormitory on result.id=dormitory.id;
Wildcard
Used to replace part of the characters in the string.
Usually used with like, and cooperate with where to complete the query.
Commonly used wildcards
%represent zero, one or more, that is, any character
_represents a single character
Query the name starting with x in the result table
select * from result where name like 'x%';
Query the name of zhangsa (any character) in the result table
select * from result where name like 'zhangsa_';
Query the name of xiao (any four characters) in the result table
select * from result where name like 'xiao____';
Match any character at the beginning, match an h, and match any character at the end
mysql> select * from result where name like '_h%';
Subquery
Also known as inner query or nested query
. It is executed before the main query, and its result will be used as the condition of the outer main query. Subqueries
can be used in addition, deletion and modification of the query.
Multi-level nested
in statement is used to determine whether a value is In the given result set
select id,name from result id where id in(1,2);
select * from result where id in(select id from result where Maths>90);
not equal to
select * from result where id!=(select id from result where name='zhangsan');
select * from result where id<>(select id from result where name='zhangsan');
Nested
select * from result where id in (select result.id from (select id from result where Maths>90)result);
update result set Maths=Maths-5 where id in (select result.id from (select id from result where Maths>60)result);
exists
select * from result where exists (select * from dormitory where room like '50_');
The following statement is executed successfully, and the previous statement is executed
NULL value
null: vacuum (nothing)
1, indicates missing value
2, is different from the number 0 or blanks (spaces)
3, use is null or is not null to judge
4, null value and empty value ('') Differences The
length of a null value is 0 and does not occupy space; the length of a null value is null, and the space occupied
is null cannot determine the null value. When a
null value uses "=" or "<>" to process the
count() calculation, null will be ignored and empty Value will be added to the calculation
Create a table
create table a(id int(3), name varchar(64), score int(3));
insert into a value(1,'zhangsan',95),(2,'lisi',NULL),(3,NULL,NULL);
Query for null values
select * from a where name is null;
Query for non-empty values
select * from a where score is not null;
Regular expression
According to the specified matching mode, match the special characters that meet the requirements in the record.
Use the regexp keyword to specify the matching mode
Common matching patterns
| character | Description |
|---|---|
| ^ | Match the beginning of the text |
| $ | Match the end character of the text |
| . | Matches any single character |
| * | Match the preceding character zero or more times |
| + | Match the preceding character one or more times |
| String | Match contains the specified string |
| p1lp2 | Match p1 or p2 |
| […] | Match any character in the character set |
| [^…] | Match any character not in brackets |
| {n} | Match the preceding string n times |
| {n，m} | Match the preceding string at least n times, as many as m times |
1. Records beginning with a specific string
select * from result where name regexp '^z';
2. Records ending with a specific string
select * from result where name regexp 'g$';
3. Records containing the specified string
select * from result where name regexp 'xiao';
4. Replace any character in the string with "."
select * from result where name regexp 'zhang...';
5. Match records containing or relationship
select * from result where name regexp 'san|si';
6. "*" matches any number of previous characters
select * from result where name regexp 'zh*';
7. "+" matches the preceding character at least once
select * from result where name regexp 'z+';
8. Match any one of the specified character set
select * from result where name regexp '^[a-z]';
Operator
There are four types of MySQL operators used to perform operations on field values in records , namely: arithmetic operators, comparison operators, logical operators, and bitwise operators
Arithmetic Operator
| Operator | description |
|---|---|
| + | addition |
| - | Subtraction |
| * | multiplication |
| / | division |
| % | Take the remainder |
select command to implement the most basic addition, subtraction, multiplication and division operations
select 1+3,4-3,2*3,6/2,5%2;
PS:
Integer division, the result is floating point.
In the division operation and the remainder operation, the divisor cannot be 0, if the divisor is 0, the returned result is NULL
Addition, subtraction, multiplication and division mixed operations
Comparison operator
| Operator | description | Operator | description |
|---|---|---|---|
| = | equal | is not null | Determine whether a value is not NULL |
| > | more than the | between and |
Between the two |
| < | Less than | in | In the collection |
| >= | greater or equal to | like | Universal character matching |
| <= | Less than or equal to | greatest | Return the maximum value for two or more parameters |
| != or <> | not equal to | least | Two or more parameters return the minimum value |
| is null | Determine whether a value is NULL | regexp | Regular expression |
1. Equal to operator
select 1=1,1=2,1='1','a'='a','a'='b','a'=null;
Comparison rules:
If both are integers, the comparison is performed according to the integer value.
If an integer is a string, the string is automatically converted to a number and then compared.
If both are strings, they are compared according to the strings.
If at least one of the two values is NULL, the result of the comparison is NULL.
2. Not equal to operator
There are two ways of writing, <> or !=, which is used to compare numbers, strings and expressions that are not equal.
If they are not equal, return 1 and if they are equal, return 0.
PS: The inequality operator cannot be used to judge NULL.
select 'ab'!='ab', 'ab'<>'abc',1!=2,1<>2,1<>null;
3. Greater than, greater than or equal, less than, less than or equal to operators
(Cannot be used to judge NULL)
select 1>2 ,2>1, 1>=2, 2>=1, 1>=null;
4. IS NULL, IS NOT NULL
IS NULL Judge whether a value is NULL, return 1 if it is NULL, otherwise return 0.
IS NOT NULL Determines whether a value is not NULL, if it is not NULL, it returns 1, otherwise it returns 0.
select 'a' is null, 'a' is not null, null is null , null is not null;
5.between and
select 3 between 1 and 3, 4 between 1 and 3, 'c' between 'a' and 'c', 'd' between 'a' and 'c';
6.least、greatest
LEAST: When there are two or more parameters, the minimum value is returned. If one of the values is NULL, the return result is NULL.
GREATEST: When there are two or more parameters, the maximum value is returned. If one of the values is NULL, the return result is NULL.
select least(1,2,3),greatest(1,2,3);
select least('a','b','c'),greatest('a','b','c');
7.in, not in
IN Determine whether a value is in the corresponding list, if it is, it returns 1, otherwise it returns 0.
NOT IN judges whether a value is not in the corresponding list, if it is not, it returns 1, otherwise it returns 0.
select 1 in (1,2,3), 'a' in ('a','b','c'), 'a' in (1,2,3);
8. Like and not like
LIKE are used to match strings. If the match is successful, it returns 1, otherwise it returns 0; NOT LIKE is the opposite of LIKE.
LIKE supports two wildcards:'%' is used to match any number of characters, and'_' can only match one character.
select 'zhangsan' like 'zhang%', 'zhangsan' like 'zhangsa_' ,'zhangsan' not like '_hangsan';
Logical Operators
| Operator | description |
|---|---|
| NOT or! | Logical negation |
| AND 或 && | Logical and |
OR或 || |
Logical OR |
| XOR | Logical exclusive OR |
1. Logic negation Logic
negation negates the logical test that follows it, turning true to false and false to true.
If the operand following NOT is 0, the resulting value is 1; if the operand is non-zero, the resulting value is 0; if the operand is NULL, the resulting value is NULL.
Note: non-zero values are all 1
select not 2, ! 0, not(1-1), ! null;
2. Logical AND
If all values are true, return 1; otherwise, return 0.
select 1 and 2, 2 && 2 , 1 and null, 1 and 0, 0 && null;
3.逻辑或（最好用or）
逻辑或表示包含的操作数，任意一个为非零值并且不是 NULL 值时，返回 1，否则返回0。
select 1 or 1, 1 or 0, 1 or null, 0 or null, 0 or 0;
4.逻辑异或
两个非 NULL 值的操作数，如果两者都是 0 或者都是非 0，则返回 0；如果一个为 0， 另一个为非 0，则返回结果为 1；
当任意一个值为 NULL 时，返回值为 NULL。
select 0 xor 1, 1 xor 1, 0 xor 0, 0 xor null;
5.总结
and运算，只要碰到0就是0，（非0和null是null）
or运算，只要碰到非0值就是1，（0和null是null）
异或运算，只要碰到null都是null
位运算符
位运算符实际上是对二进制数进行计算的运算符。
| 运算符 | 描述 |
|---|---|
| & | 按位与 |
| |
按位或 |
| ~ | 按位取反 |
| ^ | 按位异或 |
| << | 按位左移 |
| >> | 按位右移 |
select 10&15 , 10|15, 10^15, 5&~1;
按位与运算
十进制 10 & 15 #10和15按位与运算
二进制 1010 & 1111 #转换为2进制
1010
1111
————— #对应的二进制位都是 1 的，它们的运算结果为 1，否则为 0
1010
10 #1010转换为十进制
按位或运算
十进制 10 & 15 #10和15按位与运算
二进制 1010 & 1111 #转换为2进制
1010
1111
————— #对应的二进制只要有一个是1，结果就为1
1111
15 #1111转换为十进制
按位异或运算
十进制 10 & 15 #10和15按位与运算
二进制 1010 & 1111 #转换为2进制
1010
1111
————— #对应的二进制位不相同时，运算结果 1，否则为 0
0101
5 #0101转换为十进制
按位取反运算
5&~1
5 转 二进制 为 101
1 转 二进制 为 001
1 按位取反 为 110
101
110
———— #按位与运算
100
4 # 100转十进制为4
select 1<<2, 31>>3, 24>>3;
按位左移运算
1 转 十进制 为 1
左移2位后 为 100
100 转为十进制 为 4
31 转 十进制 为 11111
右移3位后 为 11 多余的位数直接删除
11 转为十进制 为 3
24 转 十进制 为 11000
右移3位后 为 11 多余的位数直接删除
11 转为十进制 为 3
常用的运算符优先级
| 优先级 | 运算符 |
|---|---|
| 1 | ！ |
| 2 | ~ |
| 3 | ^ |
| 4 | */(DIV),%(MOD) |
| 5 | +,- |
| 6 | >>,<< |
| 7 | & |
| 8 | | |
| 9 | =,<=>,>=,>,<=,<,<>,!=,IS,LIKE,REGEXP,IN |
| 10 | BETWEEN,CASE,WHEN,THEN,ELSE |
| 11 | NOT |
| 12 | &&,AND |
| 13 | ||,OR,XOR |
| 14 | := |
连接查询
创建表
create table info(id int(3), age int(3), name varchar(64), class varchar(64));
insert into info values(1,18,'zhangsan','7(1)'),(2,19,'lisi','7(1)'),(3,19,'wangwu','7(1)');
通常都是将来自两个或多个表的行结合起来，基于这些表之间的共同字段，进行数据的拼接。
要先确定一个主表作为结果集，然后将其他表的行有选择性的连接到选定的主表结果集上。
使用较多的连接查询包括：内连接、左连接和右连接
1.内连接
Use the keyword inner join in the from clause to join multiple tables, and use the on clause to set the join conditions.
Inner join is the system default table join, so the INNER keyword can be omitted after the FROM clause, and only the keyword JOIN can be used. When there are multiple tables at the same time, you can also use INNER JOIN continuously to realize the inner join of multiple tables, but for better performance, it is recommended that you do not exceed three tables.
2. Outer join
Left join, the main table is on the left, the contents of the main table will all be displayed, and the ones that are not matched in the secondary table will be displayed as NULL
select dormitory.room,info.class from dormitory left join info on dormitory.id=info.id;
Right connection, the main table is on the right, all the contents of the main table will be displayed, and the ones that are not matched in the secondary table will be displayed as NULL
select dormitory.room,info.class from dormitory right join info on dormitory.id=info.id;
