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SQL Basics – Difference between TRUNCATE, DELETE and DROP?

February 22, 2009 23 comments

DELETE and TRUNCATE are two SQL commands used to remove records from a particular table. But they differ in how they execute and operate.
 

–> DELETE: (MSDN)

1. Removes Some or All rows from a table.

2. A WHERE clause can be used to remove some rows. If no WHERE condition is specified, all rows will be removed.

3. Causes all DELETE triggers on the table to fire.

4. It removes rows row-by-row one at a time and records an entry in the Transaction logs, thus is slower than TRUNCATE.

5. Every deleted row in locked, thus it requires more number of locks and database resources.

6. According to MS BOL, if a table is a Heap or no Clustered index is defined than the row-pages emptied are not de-allocated instantly and remain allocated in the heap. Thus, no other object can reuse this associated space. Thus to de-allocate the space a Clustered index is required or TABLOCK hint should be applied in the DELETE statement.

7. This is a DML command as it is just used to manipulate/modify the table data. It does not change any property of a table.


 

–> TRUNCATE: (MSDN)

1. Removes All rows from a table.

2. Does not require a WHERE clause, so you can not filter rows while Truncating.

3. With SQL Server 2016 you can Truncate a Table Partition, for more details check [here].

4. IDENTITY columns are re-seeded on this operation, if no seed was defined then the default value 1 is used.

5. No Triggers are fired on this operation because it does not operate on individual rows.

6. It de-allocates Data Pages instead of Rows and records Data Pages instead of Rows in Transaction logs, thus is faster than DELETE.

7. While de-allocating Pages it locks Pages and not Rows, thus it requires less number of locks and few resources.

8. TRUNCATE is not possible when a table:
a. is reference by a Foreign Key or tables used in replication or with Indexed views.
b. participates in an Indexed/Materialized View.
c. published by using Transactional/Merge replication.

9. This is a DDL command as it resets IDENTITY columns, de-allocates Data Pages and empty them for use of other objects in the database.

Note: It is a misconception among some people that TRUNCATE cannot be roll-backed. But in reality both DELETE and TRUNCATE operations can be COMMITTED AND ROLL-BACKED if provided inside a Transaction. The only method to Rollback a committed transaction after DELETE/TRUNCATE is to restore the last backup and run transactions logs till the time when DELETE/TRUNCATE is about to happen.


 

–> DROP: (MSDN)

1. The DROP TABLE command removes one or more table(s) from the database.

2. All related Data, Indexes, Triggers, Constraints, and Permission specifications for the Table are dropped by this operation.

3. Some objects like Views, Stored Procedures that references the dropped table are not dropped and must be explicitly dropped.

4. Cannot drop a table that is referenced by any Foreign Key constraint.

5. According to MS BOL, Large tables and indexes that use more than 128 extents are dropped in two separate phases: Logical and Physical. In the Logical phase, the existing allocation units used by the table are marked for de-allocation and locked until the transaction commits. In the physical phase, the IAM pages marked for de-allocation are physically dropped in batches.
 


 


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DB Basics – What are DDL, DML, DCL and TCL commands & difference b/w them?

February 10, 2009 4 comments

DDL – Data Definition Language:

Statements used to Create, Alter, Drop Database Objects.

Some examples:

– CREATE: used to define new objects

– ALTER: used to modify the definition of existing objects

– DROP: used to remove existing entities.

– TRUNCATE TABLE: used to remove all rows from a table without logging the individual row deletions.

– UPDATE STATISTICS: used to update query optimization statistics on a table or indexed view.
 

DML – Data Manipulation Language:

Statements used to Retrieve, Insert, Update, Remove and Manage data within DB objects.

Some examples:

– SELECT: retrieves one or more rows from a Table or View.

– INSERT: insert one or more rows from a Table or View.

– UPDATE: changes existing data in a Table or View.

– DELETE: removes one or more rows from a Table or View.

– BULK INSERT: imports a data file into a database Table or View in a user-specified format.

– MERGE: performs Insert, Update and/or Delete operations on a Target table based on the results of a JOIN with a source table in one Transaction.

– READTEXT: reads text, ntext, or image values from a text, ntext, or image column

– UPDATETEXT: updates an existing text, ntext, or image field.

– WRITETEXT: permits minimally logged, interactive updating of an existing text, ntext, or image column.
 

DCL – Data Control Language:

Statements used to control the access of data stored in database and provide data security.

Some examples:

– GRANT: grants permissions on a securable to a principal.

– REVOKE: removes a previously granted or denied permission.

– SETUSER: allows a member of the sysadmin fixed server role or db_owner fixed database role to impersonate another user.

– EXECUTE AS, statement: sets the execution context of a session.

– EXECUTE AS, clause: define the execution context of the following user-defined modules: functions (except inline table-valued functions), procedures, queues, and triggers.

– REVERT: switches the execution context back to the caller of the last EXECUTE AS statement.

– OPEN MASTER KEY: opens the Database Master Key of the current database.

– CLOSE MASTER KEY: closes the master key of the current database.

– OPEN SYMMETRIC KEY: Decrypts a symmetric key and makes it available for use.

– CLOSE SYMMETRIC KEY: closes a symmetric key, or closes all symmetric keys open in the current session.
permission through its group or role memberships.
 

TCL – Transaction Control Language:

statements used to manage the changes made by DML statements. It allows statements to be grouped together into logical transactions.

Some Examples:

– BEGIN DISTRIBUTED TRANSACTION: specifies the start of a Transact-SQL distributed transaction managed by Microsoft Distributed Transaction Coordinator (MS DTC).

– BEGIN TRANSACTION: marks the starting point of an explicit, local transaction. BEGIN TRANSACTION increments @@TRANCOUNT by 1.

– COMMIT TRANSACTION: marks the end of a successful implicit or explicit transaction.

– COMMIT WORK: marks the end of a transaction.

– ROLLBACK TRANSACTION: rolls back an explicit or implicit transaction to the beginning of the transaction, or to a savepoint inside the transaction.

– ROLLBACK WORK: rolls back a user-specified transaction to the beginning of the transaction.

– SAVE TRANSACTION: sets a savepoint within a transaction.
 

Check the video:

DDL DML DCL TCL
 


An introduction to SQL and its Components

February 3, 2009 Leave a comment

–>Introduction:

SQL or Structured Query Language is a language that provides an interface to relational database systems. The proper pronunciation of SQL is “ess cue ell,” and not “sequel” as is commonly heard. SQL was developed by IBM in the 1970s for use in System R, and is a de facto standard, as well as an ISO and ANSI standard. In common usage SQL also encompasses DML (Data Manipulation Language), for INSERTs, UPDATEs, DELETEs and DDL (Data Definition Language), used for creating and modifying tables and other database structures. The development of SQL is governed by standards. A major revision to the SQL standard was completed in 1992, called SQL2. SQL3 support object extensions and are (partially?) implemented in Oracle8 and 9i.

SQL (Structured Query Language) is a database computer language designed for managing data in relational database management systems (RDBMS). Its scope includes data query and update, schema creation and modification, and data access control. SQL was one of the first languages for Edgar F. Codd’s relational model in his influential 1970 paper, “A Relational Model of Data for Large Shared Data Banks” and became the most widely used language for relational databases.
 

–>Language elements:

The SQL language is sub-divided into several language elements, including:

1. Clauses which are in some cases optional, constituent components of statements and queries.

2. Expressions which can produce either scalar values or tables consisting of columns and rows of data.

3. Predicates which specify conditions that can be evaluated to SQL three-valued logic (3VL) Boolean truth values and which are used to limit the effects of statements and queries, or to change program flow.

4. Queries which retrieve data based on specific criteria.

5. Statements which may have a persistent effect on schemas and data, or which may control transactions, program flow, connections, sessions, or diagnostics.

6. SQL statements also include the semicolon (“;”) statement terminator. Though not required on every platform, it is defined as a standard part of the SQL grammar.

7. Insignificant whitespace is generally ignored in SQL statements and queries, making it easier to format SQL code for readability.
 

–> Queries:

The most common operation in SQL is the query, which is performed with the declarative SELECT statement. SELECT retrieves data from one or more tables, or expressions. Standard SQL statements have no persistent effects on the database. Some non-standard implementations of SELECT can have persistent effects, such as the SELECT INTO syntax that exists in some databases.

Queries allow the user to describe desired data, leaving the database management system (DBMS) responsible for planning, optimizing, and performing the physical operations necessary to produce that result as it chooses.

A Query includes a list of columns to be included in the final result immediately following the SELECT keyword. An asterisk (“*”) can also be used to specify that the query should return all columns of the queried tables. SELECT is the most complex statement in SQL, with optional keywords and clauses that includes:

1. The FROM clause which indicates the table(s) from which data is to be retrieved. The FROM clause can include optional JOIN subclauses to specify the rules for joining tables.

2. The WHERE clause includes a comparison predicate, which restricts the rows returned by the query. The WHERE clause eliminates all rows from the result set for which the comparison predicate does not evaluate to True.

3. The GROUP BY clause is used to project rows having common values into a smaller set of rows. GROUP BY is often used in conjunction with SQL aggregation functions or to eliminate duplicate rows from a result set. The WHERE clause is applied before the GROUP BY clause.

4. The HAVING clause includes a predicate used to filter rows resulting from the GROUP BY clause. Because it acts on the results of the GROUP BY clause, aggregation functions can be used in the HAVING clause predicate.

5. The ORDER BY clause identifies which columns are used to sort the resulting data, and in which direction they should be sorted (options are ascending or descending). Without an ORDER BY clause, the order of rows returned by an SQL query is undefined.
 

–> A simple SELECT example:

SELECT *
FROM dbo.Book
WHERE price > 100.00
ORDER BY title;

 

–> With SQL you can:

– Create new Databases.
– Create new Tables in a database.
– Execute Queries against a database.
– Insert records in a database.
– Update records in a database.
– Delete records from a database.
– Retrieve data from a database.
– Create stored procedures in a database.
– Create views in a database.
– Set permissions on tables, procedures, and views.
 

For the complete history check my following [blog post].