Sql commands sql objects and access to data are

SQL commands
SQL objects and access to data are managed through SQL commands. Standard SQL
supports a relatively small command set. You might think this would make SQL easy to
use, but this is not necessarily the case. Options and conditional clauses can cause
individual command strings to become quite extensive. To the uninitiated, they can be
somewhat cryptic. For example:
SELECT mail.ship_code, mail.ship_date, count (mail.resp_code)
FROM mail
WHERE resp_code <> 0000
GROUP BY mail.ship_code, mail.ship_date
ORDER BY mail.ship_date
In most cases, the best way to make a SQL database available to end-users is through a
client application that makes queries more user-friendly. Another common access
method is through the Internet, specifically through Web browsers. SQL Server gives
you the option of generating Extensible Markup Language (XML) output.
Transact-SQL (T-SQL)
Any time a command is passed to Microsoft SQL Server, it is done so using Transact-
SQL, even when using the graphic management and administration tools, though you
might not realize it. Graphic tools take the action you want to perform, such as creating
a table, and generate the necessary T-SQL statements for you.
There are three basic categories of T-SQL statements:
Data definition — Used to create and modify databases
Data manipulation — Used to enter, modify, and extract data
Data control — Used to manage access to SQL Server data.
T-SQL statements include control-of-flow statements, local variables, and other features
that let you write batches, short programs, and scripts (collections of batches).

SQL Server fundamentals
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T-SQL and ANSI-92 (SQL-92)
T-SQL is SQL-92-compliant but includes features and functionality not provided
through SQL-92. This means that scripts written for SQL-92 will run unmodified on the
SQL Server. This allows for portability between different database management
systems.
The converse is not true. Scripts written employing T-SQL-specific statements will not
run on other SQL-92-compliant systems.
Database introduction
In the SQL model, data is stored in databases. SQL databases organize data in tables.
Tables contain rows that represent data records and columns that represent data field
values, as shown in Exhibit 1-1.
Exhibit 1-1: SQL table model
Each column has a data type , defining the format in which data is stored in that field.
Data types can be a fixed length, meaning that they are always the same size, or variable
length, changing to fit the data they hold. Columns can be further defined to set default
values and rules for data integrity.
One value, called the primary key , identifies each row in a table as unique. This key will
contain one or more columns representing a unique, non-null value.
You can also identify the columns used for sorting and organizing data. These are called
indexes. Indexes help the SQL server quickly locate records containing a specified
index value or falling within a range of values.