SQL (pronounced "ess-que-el") stands for Structured Query Language. SQL is used to
communicate with a database. According to ANSI, it is the standard language for relational
database management systems. SQL statements are used to perform tasks such as update data on
a database, or retrieve data from a database. Some common relational database management
systems that use SQL are:Oracle, Sybase, Microsoft SQL Server, Access, Ingres, etc. [1]Although
most database systems use SQL, most of them also have their own additional proprietary
extensions that are usually only used on their system. However, the standard SQL commands
such as "Select", "Insert", "Update", "Delete", "Create", and "Drop" can be used to accomplish
almost everything that one needs to do with a database.
A relational database system contains one or more objects called tables. The data or
information for the database are stored in these tables. Tables are uniquely identified by their
names and are comprised of columns and rows. Columns contain the column name, data type,
and any other attributes for the column. Rows contain the records or data for the columns. Here is
a sample table called "weather".
City, state, high, and low are the columns. The rows contain the data for this table:

The select statement is used to query the database and retrieve selected data that match the
criteria that you specify. Here is the format of a simple select statement:
select "column1"
[,"column2",etc]
from "tablename"
[where "condition"];
[] = optional

The column names that follow the select keyword determine which columns will be returned
in the results. You can select as many column names that you'd like, or you can use a "*" to
select all columns.
The table name that follows the keyword from specifies the table that will be queried to
retrieve the desired results.
The where clause (optional) specifies which data values or rows will be returned or
displayed, based on the criteria described after the keyword where.
The create table statement is used to create a new table. Here is the format of a simple create
table statement:
create table "tablename"
("column1" "data type",
"column2" "data type",
"column3" "data type");
Format of create table if you were to use optional constraints:
create table "tablename"
("column1" "data type"
[constraint],
"column2" "data type"
[constraint],
"column3" "data type"
[constraint]);
[ ] = optional
Note: You may have as many columns as you'd like, and the constraints are optional.
Example:
create table employee
(first varchar(15),
last varchar(20),
age number(3),
address varchar(30),
city varchar(20),
state varchar(20));
[2]To create a new table, enter the keywords create table followed by the table name,
followed by an open parenthesis, followed by the first column name, followed by the data type
for that column, followed by any optional constraints, and followed by a closing parenthesis. It is
important to make sure you use an open parenthesis before the beginning table, and a closing
parenthesis after the end of the last column definition. Make sure you separate each column
definition with a comma. All SQL statements should end with a ";".
The table and column names must start with a letter and can be followed by letters, numbers,

or underscores ê not to exceed a total of 30 characters in length. Do not use any SQL reserved
keywords as names for tables or column names (such as "select", "create", "insert", etc).
Data types specify what the type of data can be for that particular column. If a column
called "Last_Name", is to be used to hold names, then that particular column should have a
"varchar" (variable-length character) data type.
Note:Here are the most common data types:
char(size) Fixed-length character string. Size is specified in parenthesis. Max 255 bytes.
varchar(size) Variable-length character string. Max size is specified in parenthesis.
number(size) Number value with a max number of column digits specified in parenthesis.
Date Date value
number(size,d)
Number value with a maximum number of digits of "size" total, with a
maximum number of "d" digits to the right of the decimal.
What are constraints? When tables are created, it is common for one or more columns to
have constraints associated with them. A constraint is basically a rule associated with a column
that the data entered into that column must follow. For example, a "unique" constraint specifies
that no two records can have the same value in a particular column. They must all be unique. The
other two most popular constraints are "not null" which specifies that a column can't be left blank,
and "primary key". A "primary key" constraint defines a unique identification of each record (or
row) in a table.
The insert statement is used to insert or add a row of data into the table.
To insert records into a table, enter the key words insert into followed by the table name,
followed by an open parenthesis, followed by a list of column names separated by commas,
followed by a closing parenthesis, followed by the keyword values, followed by the list of values
enclosed in parenthesis. The values that you enter will be held in the rows and they will match up
with the column names that you specify. Strings should be enclosed in single quotes, and
numbers should not.
insert into "tablename"
(first_column,...last_column)
values (first_value,...last_value);
In the example below, the column name first will match up with the value 'Luke', and the
column name state will match up with the value 'Georgia'.
Example:
insert into employee
(first, last, age, address, city, state)
values ('Luke', 'Duke', 45, '2130 Boars Nest',
'Hazard Co', 'Georgia');
Note: All strings should be enclosed between single quotes: 'string'

Updating Records
The update statement is used to update or change records that match a specified criteria.
This is accomplished by carefully constructing a where clause.
update "tablename"
set "columnname" =
"newvalue"
[,"nextcolumn" =
"newvalue2"...]
where "columnname"
OPERATOR "value"
[and|or "column"
OPERATOR "value"];
[ ] = optional
Examples:
update phone_book
set area_code = 623
where prefix = 979;
update phone_book
set last_name = 'Smith', prefix=555, suffix=9292
where last_name = 'Jones';
update employee
set age = age+1
where first_name='Mary' and last_name='Williams';
Deleting Records
The delete statement is used to delete records or rows from the table.
delete from "tablename"
where "columnname"
OPERATOR "value"
[and|or "column"
OPERATOR "value"];
[ ] = optional
Examples:
delete from employee;
Note: if you leave off the where clause, all records will be deleted!
delete from employee
where lastname = 'May';
delete from employee
where firstname = 'Mike' or firstname = 'Eric';

To delete an entire record/row from a table, enter "delete from" followed by the table name,
followed by the where clause which contains the conditions to delete. If you leave off the where
clause, all records will be deleted.
The drop table command is used to delete a table and all rows in the table.
To delete an entire table including all of its rows, issue the drop table command followed by
the table name. Drop table is different from deleting all of the records in the table. Deleting all of
the records in the table leaves the table including column and constraint information. Dropping
the table removes the table definition as well as all of its rows.
drop table "tablename"
Example:
drop table myemployees_ts0211;

1, clause  [klɔ:z]
n. 分句,从句,条款,款项

2, parenthesis  [pə'renθisis]
n. 括弧,插入语,附带
3, criteria 
n. 标准