SQL Statement Basics

Follow the http://www.w3school.com.cn/sql/sql_syntax.asp tutorial to learn and make notes here.

Grammar:

SQL is not case sensitive!

If you are using MS Access and SQL Server 2000, you do not have to use semicolons after each SQL statement, although some database software requirements must use semicolons.

SQL can be divided into two parts: Data manipulation Language (DML) and data definition language (DDL).

SQL (Structured Query language) is the syntax for executing queries. However, the SQL language also contains syntax for updating, inserting, and deleting records.

The query and update Directives form the DML portion of SQL:

• • SELECT-Get data from a database table
  • Update-updating data in a database table
  • Delete-Deletes data from the database table
  • INSERT into-inserts data into a database table

The Data definition language (DDL) portion of SQL gives us the ability to create or delete tables. We can also define indexes (keys), specify links between tables, and impose constraints between tables.

The most important DDL statement in SQL:

• • Create database-Creating new databases
  • ALTER DATABASE-Modify databases
  • CREATE table-Creates a new table
  • ALTER TABLE-Change (change) database table
  • drop table-Delete tables
  • Create index-Creating indexes (search key)
  • Drop INDEX-Delete indexes

SELECT column name from table name

*From Persons

DISTINCTCompany from Orders      #关键词 DISTINCT is used to return only different values.

WHERE column operator values

WHERE City=‘Beijing‘     #例子中的条件值周围使用的是单引号。SQL 使用单引号来环绕文本值（大部分数据库系统也接受双引号）。如果是数值，请不要使用引号。

operator	Description
=	Equals
<>	Not equal to, in some versions of SQL, the operator <> can be written as! =.
>	Greater than
<	Less than
>=	Greater than or equal
<=	Less than or equal
Between	Within a range
Like	Search for a pattern

Text value:

FirstName=‘Bush‘FirstName=Bush

Numerical:

Year>1965Year>‘1965‘

And and or can combine two or more conditions in a where sub-statement. If both the first condition and the second condition are true, the AND operator displays a record. If only one of the first and second conditions is true, the OR operator displays a record.

(ORFirstname= ' William '   AND  lastname= ' Carter '    #可以把 and and or together (using parentheses to form complex expressions)

The order BY statement is used to sort the result set based on the specified column. The order BY statement sorts records by default in ascending order. If you want to sort records in descending order, you can use the DESC keyword.

ORDER BYDESCASC   #以逆字母顺序显示公司名称，并以数字顺序显示顺序号

INSERT INTO statement inserts a new row into the table

INSERT into table name values (value 1, value 2,....)

INSERT into Persons VALUES (' Gates ', ' Bill ', ' xuanwumen ', ' Beijing ')

You can also specify the columns for which you want to insert data:

INSERT into table_name (column 1, column 2,...) Values (value 1, value 2,....)

INSERT into Persons (LastName, Address) VALUES (' Wilson ', ' champs-elysees ')

The Update statement is used to modify the data in the table.

UPDATE table name SET column name = new value WHERE Column name = value

UPDATE person SET address = ' Zhongshan. ', city = ' nanjing ' WHERE LastName = ' Wilson '     #修改地址 (address), and add the town name

Delete statement to remove rows from a table

DELETE From table name WHERE column name = value

DELETE from table_name      #在不删除表的情况下删除所有的行. This means that the structure, properties, and indexes of the table are complete

Or:

DELETE * FROM table_name      #在不删除表的情况下删除所有的行. This means that the structure, properties, and indexes of the table are complete

SQL Statement Basics