Table of contents
insert into table name value(data, data),.....;
insert into table name (column 1, column 2.....) value(data, data),.....;
How to insert data of datatime type?
select column 1, column 2... from table name;
The query field is an expression
Aliases the query result columns
Note: Execution order of select conditional query
delete from table name where condition;
update table name set (column name = value), (column name = value).... where condition;
First create a table named: title: all operations below are based on this table
Note: BecauseMySQL is not case-sensitive so both uppercase and lowercase are acceptable.
New
insert into Table name values/value(number position, number position),.... ..；
Can be inserted in single line or multiple lines.
insert into Table name (column 1, column 2....) value( number position, number position),.......;
Specify column insertion, can be inserted in single row or multiple rows.
How to insert data of datatime type?
You can use afixed formatstring to represent the date
You can also use the now() function to obtain the current time
Add new table field
alter table table name add field name and type
The fields of the table after insertion are:
Inquire
select * from 表名；
Full column query Display all data in the table
* display pass mark possibleindex possessive sequence
select 列1,列2...... from 表名；
Specify column query
QueryThe field is an expression
- QueryMinus 20 points for Chinese language score
- Check each subjectThe total score
Note:The last two rows are null becauseIn MySQl, the result of null and any value is null
Give an alias for the query result column
select expression/column name as alias from Table name;
Left weight:DISTINCT
select distinct single column/multiple columns from table name;
willremoveduplicates from the query results (retaining only one item)
排序：ORDER BY
select * from 表名 order by 列名 asc/desc；
- ASC is ascending order (from small to large)
- DESC is descending order (from large to small)
- Default is ASC
Column name can be single orcan be multiple
例：select * from 表名 order by A，B ；
Sort by B if A is equal
select * from 表名 order by A asc，B desc；
Arrange according toA column in ascending order and B column in descending order
Specify a column to be sorted in ascending/descending order
null is considered the minimum value, but only in sorting.
Conditional query
select * from table namewhere expression/condition (cannot be an alias )
Display data that meets the conditions
comparison operator
| operator | illustrate |
| >, >=, <, <= | Greater than, greater than or equal to, less than, less than or equal to |
| = | Equal to, NULL is unsafe, for example, the result of NULL = NULL is NULL -> false |
| <=> | Equal to, NULL is safe, for example, NULL <=> The result of NULL is TRUE(1) |
| !=, <> | not equal to |
| BETWEEN a0 AND a1 |
range matching, [a0, a1], closed interval, if a0 <= value <= a1, return TRUE(1) |
| IN (option, ...) | If it is any one of options, return TRUE(1) |
| IS NULL | is NULL |
| IS NOT NULL | Not NULL |
| LIKE ‘..%../.._..’ | Fuzzy matching. % represents any number (including 0) of any character; _ represents any character character |
LIKE ‘..%../.._..’ usage example:
select * from table name where column name like 'Sun%';
Find records in the specified column that meet the conditions behind like.
- ‘%Sun’: The last subcharacter of the string is ‘Sun’
- ‘Sun%’: The first character of the string is ‘Sun’
- ‘%Sun%’: The string contains ‘Sun’
Logical Operators
| operator | illustrate |
| AND | Multiple conditions must all be TRUE(1) for the result to be TRUE(1) |
| OR | If any condition is TRUE(1), the result is TRUE(1) |
| NOT | The condition is TRUE(1), the result is FALSE(0) |
Example: Meet people whose math score is greater than 80
Note: Execution order of select conditional query
- Iterate through each record in the table
- Bring the value of the current record into the condition and filter based on the condition
- If this record meets the conditions, retain it and evaluate the expression on the column.
- If there is order by, all results will be sorted after all rows have been obtained (the expression has been calculated).
Becausethe third step is to define the alias, and where is executed in the second step< /span>So an error will be reported;
Pagination query: LIMIT
select * from table name limit the number of rows to be queried;
select * from table name limit the number of rows to be queried offset offset ('subscript'from 0 start);
delete
delete from table name;
Deletes all data in this table, but does not delete the table.
delete from table name where condition;
Revise
update table nameset column name=value, column name=value.... where Condition;
You can modify one column or multiple columns.
The where condition here is to limit which values can be modified.
Modify one column at a time:
Modify multiple columns at once:
Numeric types in MySQL
| type of data | size | illustrate |
| BIT[ (M) ] | M-designated position number, black 为1 |
Binary number, M ranges from 1 to 64, stores value range from 0 to 2^M-1 |
| TINYINT | 1 byte | Equivalent to byte in JAVA language |
| SMALLINT | 2 bytes | Equivalent to short in JAVA language |
| INT | 4 bytes | |
| BIGINT | 8 bytes | Equivalent to Long in JAVA language |
| FLOAT(M, D) | 4 bytes | Single precision, M specifies the length, D specifies the number of decimal places. Loss of precision will occur |
| DOUBLE(M, D) |
8 bytes | Double precision, M specifies the length, D specifies the number of decimal places. Loss of precision will occur |
| DECIMAL(M, D) |
M/D maximum 值+2 |
Double precision, M specifies the length, D represents the number of decimal places. Exact value |
| NUMERIC(M, D) |
M/D maximum 值+2 |
Same as DECIMAL |
Character types in MySQL:
| type of data | size | illustrate |
| VARCHAR (SIZE) | 0-65,535 bytes | Variable length string, size is the character length |
| TEXT | 0-65,535 bytes | long text data |
| MEDIUMTEXT | 0-16 777 215 bytes | Medium length text data |
| BLOB | 0-65,535 bytes | Long text data in binary form |
Date types in MySQL:
| type of data | size | illustrate |
| DATETIME | 8 bytes | The range is from 1000 to 9999, and no time zone retrieval and conversion will be performed . |
| TIMESTAMP | 4 bytes | Ranges from 1970 to 2038, automatically retrieves the current time zone and converts. |
Constraint types in MySQL
| constraint type | illustrate | Example |
| NULL constraint | Use NOT NULL to specify that the column is not empty |
name varchar(20) not null, |
| UNIQUE unique constraint | Specify columns that are unique and non-duplicate | name varchar(20) unique, |
| DEFAULT default value is about bundle |
Specifies the default value when the column is empty | age int default 20, |
| primary key constraints | NOT NULL and UNIQUE combined |
id int primary key, |
| foreign key constraints | Relate other tables’ primary or unique keys | foreign key (field name) references main table (column) |
| CHECK promise (completed solution) |
Guarantee that the values in the column meet the specified conditions |
check (sex ='male' or sex='female') |