MYSQL_ Chapter III

Chapter III : advanced Query (-) 3.1: Modifying a table

3.1.1: Modifying a table

Syntax :

Alter table < old table name > Rename [to] < new table name >;

example : Alter table ' demo01 ' rename ' demo02 ';

3.1.2: Adding fields

Syntax :

Alter table name ADD field name data type [ properties ];

example : Alter Table ' Demo02 ' ADD ' passWord ' varchar (not NULL);

3.1.3: Modifying Fields

Syntax :

Alter table name change original field name new field name data type [ properties ];

% Modified field name %

Alter Table ' Demo02 ' change ' name ' username ' char (ten) not NULL;

3.1.4 Delete a field

Deleting a field removes a field from the table in the data table :

syntax : ALTER table name DROP field name ;

3.1.5 Adding a primary foreign key

Add a PRIMARY KEY constraint

syntax : ALTER table name ADD CONSTRAINT primary Key name PRIMARY key table name ( primary key field );

Add a FOREIGN KEY constraint

syntax : ALTER table name ADD CONSTRAINT foreign key name FOREIGN key ( foreign key field ) REFERENCES The associated table name ( Association field );

3.2 DML statement review

3.2.1 inserting data records

Inserting a single row of data

syntax : INSERT into table name [( field List )] VALUES ( List of values );

Inserting multiple rows of data

syntax : INSERT into new table ( field list ) VALUES ( Value list 1), ( Value List 2), ..., ( Value list n);

Note:in the YesINSERTstatement when you insert a record,If the field name is not included, VALUESThe order of the fields in the value list must be the same as the order of the fields in the table definition,If the table structure changes(If you performed the Add data operation) the list of values also needs to change,Otherwise there will be an error.If an inserted field name is specified,would have avoided the problem.,so,It is recommended that you specify a specific field name when inserting data.

Inserting a query structure into a new table

Syntax 1:

INSERT into new table ( field 1, field 2,......)

SELECT Field 1, field 2,......

From the original table ;

Syntax 2:

CREATE table New Table (SELECT field 1, field 2, ...). from the original table );

3.2.2 updating data records

syntax : UPDATE table name SET field 1= value 1, field 2= Value 2,........ Field n= value N [WHERE condition ];

3.2.3 deleting data records

syntax : DELETE from table name [WHERE condition ];

syntax : TRUNCATE table name ;

3.3 data query

3.3.1 DQL Statement Review

data query statements in SQL Server :

syntax : select< List of field names >

from< table name or view >

[WHERE < query conditions ;]

[GROUP BY < Group field name ;]

[Having < conditions ;]

[ORDER BY < sorted field name > [ASC or DESC]];

3.3.2 LIMIT clause

syntax : select< List of field names >

from< table name or view >

[WHERE < query conditions ;]

[GROUP BY < Group field name ;]

[ORDER BY < sorted field name > [ASC or DESC]]

[LIMIT [ position offset ,] number of rows ];

of which : 1. position offset refers to starting from the first few data in the result set ( The position offset of the first record is 0, and section 2 is 1,....), and is displayed when omitted from the first record .

Row value shows the number of records in the record

3.3.3 Common Functions

Common aggregate functions :

AVG ()	Returns the average of a field
COUNT ()	Returns the number of rows in a field
MAX ()	Returns the maximum value of a field
MIN ()	Returns the minimum value of a field
SUM ()	Returns the and of a field
Name of function	Role

Commonly used String functions

Name of function	Role	Example
CONCAT (str1,str2,..., strn)	Connection string Span style= "Font-family:calibri" >str1,str2,..., strn	Select CONCAT (' My ', ' S ', ' QL '); Return :mysql
INSERT (str,pos,len,newstr)	String Span style= "Font-family:calibri" >str from pos position start ,len characters long substring replaced with string newstr	SELECT INSERT (' This is sql SERVER database ', 3,10, ' MySQL '); Return : This is mysql database
LOWER (str)	turns all characters in the string str to lowercase
UPPER (str)	capitalize all strings in the string str	SELECT UPPER (' MySQL '); Back to : MYSQL
SUBSTRING (Str,num,len)	Returns a substring of the first num position of the string str with a length of Len

3. time-Date function

Name of function	Role	Example
Curdate ()	Get Current date	SELECT curdate ();
Curtime ()	Get current time
Now ()	Get the current date and time
WEEK (date)	Return Date is the week ordinal of a year
Year (date)	Returns the year of the date
HOUR (Time)	Returns the hour value of time
MINUTE (Time)	Returns the minute value of time
DATEDIFF (DATE1,DATE2)	Returns the number of days separated by the date parameter date1 and date2
Adddate (Date,n)	Calculate date parameter date plus n days of day

numeric functions

Function name	Effect	example
Ceil (x)	" Returns the smallest integer greater than or equal to the value x	SELECT Ceil (2.3) Return :3
Floor (x)	Returns less than or equal to the value x
RAND ()	return 0~1

3.4 Sub-query

3.4.1 Simple subquery

syntax : SELECT ... From table 1 WHERE field 1 comparison operator ( subquery );

Use As keyword , ANSI compliant

The alias of the SELECT field List from the table name as table ;

a simple way to use spaces

The alias of the SELECT field list from the table name table ;

3.5 in and not in subqueries

3.5.1 in sub-query

working with sub-queries when multiple records are returned for a result , the = of the condition is changed to In

The in subquery can follow a subquery that returns multiple records to detect whether a field's value exists in a range .

3.5.2 Not in sub-query

not in other data within the range of the subquery .

MYSQL_ Chapter III