SQL statement Additions, deletions, and subqueries

Modify Table Modification Table

Grammar:

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

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

Add Field

Grammar:

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

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

modifying fields

Grammar:

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;

Delete a field

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

Syntax: ALTER table name DROP field name;

Add primary foreign key

1. Add a PRIMARY KEY constraint

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

2. Add a foreign KEY constraint

Syntax: ALTER table name ADD CONSTRAINT foreign key name FOREIGN key (foreign key field) REFERENCES associated table name (associated field);

DML statement review Insert data record

1. Inserting a single row of data

Syntax: INSERT into table name [(field list)] VALUES (list of values);

2. Inserting multiple rows of data

Syntax: INSERT into new Table (field list) VALUES (Value List 1), (Value List 2), ..., (value list n);

Note: If you do not include the field name in the INSERT statement when inserting a record, the order of the fields in the values list must be the same as the order of the fields in the table definition, or an error will occur if the table structure changes (such as adding a data operation). If you specify an inserted field name, This problem is avoided, so it is recommended that you specify a specific field name when inserting data.

3. 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);

Update data records

Syntax: UPDATE table name SET field 1= value 1, field 2= value 2,........ field n= value n [WHERE condition];

Delete data records

Syntax: DELETE from table name [where condition];

Syntax: TRUNCATE table name;

Data Query 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]];

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];

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

2. Row values show the number of records

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 STR1,STR2,..., strn as a full string	SELECT CONCAT (' My ', ' S ', ' QL '); Back to: MySQL
INSERT (STR,POS,LEN,NEWSTR)	Starts the string str from the POS position, and the Len characters are replaced with a string newstr	SELECT INSERT (' This is SQL Server database ', 3,10, ' MySQL '); Back: This is the 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

4. Numeric functions

Name of function	Role	Example
Ceil (x)	Returns the smallest integer greater than or equal to the number X	SELECT Ceil (2.3) Returns: 3
Floor (x)	Returns the largest integer less than or equal to the number X
RAND ()	Returns the random number between 0~1

Sub-query Simple subquery

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

Use the AS keyword to comply with ANSI standards

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;

In and not in subquery in sub-query

Use a subquery when there are multiple records in the returned 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.

Not in sub-query

Other data that is not in the in subquery scope.

SQL statement Additions, deletions, and subqueries