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Wednesday, February 24, 2010

Understanding the Difference Between SQL and a Programming Language

To solve problems in a procedural programming language (such as Basic, C, COBOL, FORTRAN, and so on), you write lines of code that perform one operation after another until the program completes its tasks. The program may execute its lines of code in a linear sequence or loop to repeat some steps or branch to skip others. In any case, when writing a program in a procedural language, the programmer specifies what is to be done and how to do it.

SQL, on the other hand, is a nonprocedural language in that you tell SQL what you want to do without specifying exactly how to accomplish the task. The DBMS, not the programmer, decides the best way to perform the job. Suppose, for example, that you have a CUSTOMER table and you want a list of customers that owe you more than $1,000.00. You could tell the DBMS to generate the report with this SQL statement:

  SELECT
NAME, ADDRESS, CITY, STATE, ZIP, PHONE_NUMBER,
BALANCE_DUE
FROM
CUSTOMER
WHERE
BALANCE_DUE > 1000.00

If writing a procedural program, you would have to write the control loop that reads each row (record) in the table, decides whether to print the values in the columns (fields), and moves on to the next row until it reaches the end of the table. In SQL, you specify only the data you want to see. The DBMS then examines the database and decides how best to fulfill your request.

Although it is an acronym for "Structured Query Language," SQL is more than just a data retrieval tool. SQL is a:

  • Data definition language (DDL), for creating (and dropping) database objects such as tables, constraints, domains, and keys.

  • Data manipulation language (DML), for changing values stored in columns, inserting new rows, and deleting those you no longer want.

  • Data control language (DCL), for protecting the integrity of your database by defining a sequence of one or more SQL statements as a transaction in which the DBMS must complete all statements successfully or have none of them affect the database. DCL also lets you set up the security structure for the database.

  • Query language, for retrieving data.

In addition to the DDL, DML, DCL, and query functions, SQL maintains data integrity and coordinates concurrent access to the database objects. In short, SQL provides all of the tools you need for controlling and interacting with the DBMS.

Despite all that it does, SQL is not a complete computer language (like Basic, C, or FORTRAN) because it contains no block (BEGIN, END) statements, conditional (IF) statements, branch (GOTO) statements, or loop (DO, WHILE, FOR) statements. Because it lacks input statements, output statements, and common procedural language control methods, SQL is considered a data sublanguage. What SQL lacks in procedural language components, it makes up for in the database realm with statements specialized for database management and data retrieval tasks.

You can get information from an SQL database by submitting ad hoc queries during an interactive session or by embedding SQL statements in a procedural application program. Issuing queries during an interactive session is most appropriate when you want a quick answer to a specific question that you may ask only once. If, on the other hand, you need the same information repeatedly and want to control the format of the output, embedding SQL statements in an application program or having the program send SQL commands to the DBMS via a call-level interface makes the most sense.


Note

Most major database vendors are adding procedural programming language-like features to their SQL products by allowing you to create stored procedures. Stored procedures are sequences of SQL statements that you tell the DBMS to execute by entering the stored procedure's name at the console during an interactive session, or by sending the name as a command to the DBMS within an application program. The stored procedure itself contains SQL statements and code written in the vendor's extensions to SQL that provide procedural language facilities such as BEGIN-END blocks, IF statements, functions, procedures, WHILE loops, FOR loops, and so on. Oracle, for example, extends SQL with PL/SQL and SQL *Plus, while Microsoft lets you use its Transact-SQL extensions in stored procedures.

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