In standard SQL definitions, DML is defined as CRUD-operations:
SELECT
INSERT
UPDATE
DELETE
Call, explain, merge, etc...
Within DML, is there a further distinction between what might be the 'query' (retrieving information) and what might be 'modification' (modifying information) ? For example, something like:
DQL (query)
SELECT
DWL (write)
INSERT, UPDATE, DELETE
Or does no such distinction exist and people just use the terms 'read' and 'write' ? Another good reference is: What are DDL and DML?.
Related
Is the BEFORE trigger statement executed before DDL statements or is it inside a DDL statement?
I'm building a small DBMS and currently developing the DDL, and I'm confused about handling the BEFORE statement.
Here's not a defined behaviour for it:
https://web.csulb.edu/colleges/coe/cecs/dbdesign/dbdesign.php?page=sql/ddldml.php
Oracle describes the behaviour but not what can come inside the preceding or following trigger or whether they can be DDL statements or not:
https://www.oracletutorial.com/plsql-tutorial/oracle-trigger/
"BEFORE" is part of the DDL statement that defines the trigger. It describes when the trigger fires in relation to a DML command on a table: i.e. BEFORE or AFTER the change to the data is actually made. This allows you to validate or modify the new data as it is being applied, or to add or modify dependent data like a child table after the parent table update is complete.
While you can technically embed DDL into the body of a trigger using "execute immediate", it is typically considered very bad practice to do so. Is there a specific use case you are considering for including DDL statements in a trigger?
I am currently reading the offical Microsoft book 'Database Administration Fundamentals' in preparation to sitting it's exam.
I understand what DDL and DML are but Microsoft show DELETE as being both a DDL and DML statement. I have googled this but I cannot anything that confirms or denies this.
A good reference to this is the question: What is DDL and DML Which shows it as a DML. Below is the segments from the book:
Data Manipulation Language (DML) is the language element that allows
you to use the core statements INSERT, UPDATE, DELETE, and MERGE to
manipulate data in any SQL Server tables. Core DML statements include
the following: • SELECT: Retrieves rows from the database and enables
the selection of one or many rows or columns from one or many tables
in SQL Server. • INSERT: Adds one or more new rows to a table or a
view in SQL Server. • UPDATE: Changes existing data in one or more
columns in a table or view. • DELETE: Removes rows from a table or
view. • MERGE: Performs insert, update, or delete operations on a
target table based on the results of a join with a source table.
the six main DDL statements are as follows: • USE: Changes the
database context. • CREATE: Creates a SQL Server database object
(table, view, or stored procedure). • ALTER: Changes an existing
object. • DROP: Removes an object from the database. • TRUNCATE:
Removes rows from a table and frees the space used by those rows. •
DELETE: Remove rows from a table but does not free the space used by
those rows removed.
Is the book out of date/ wrong. Can someone help shed light on this I see conflicting lists of what are the full DDL and DML statements.
I agree with you, DELETE is DML. Moreover, I dare say, TRUNCATE should also be considered DML, since logically is equivalent to a DELETE statement. The fact that TRUNCATE is a DROP and CREATE is not enough in my opinion to justify assigning it to DDL, since the two together, carried out as one atomic operation, do not affect the schema of the database.
there is an specify remark for SELECT INTO clause,that I don't know it?is SELECT INTO a DDL or DML?I will appreciate if explain me that specify remark?
thanks
I would say DML as DDL is used to define the database structure, and DML for managing data.
Select into is not different from a insert into, I belive.
Both. It's DDL because it changes the catalog. It's DML because SELECT is DML.
The Wikipedia article on Data manipulation language says (highlighting is mine):
The SQL-data change statements are a subset of the SQL-data statements; this also contains the SELECT query statement, which strictly speaking is part of the DQL, not the DML. In common practice though, this distinction is not made and SELECT is widely considered to be part of DML, so the DML consists of all SQL-data statements, not only the SQL-data change statements. The SELECT ... INTO ... form combines both selection and manipulation, and thus is strictly considered to be DML because it manipulates (i.e. modifies) data.
I Think Basically Select into is a combination query provided by microsoft from Sql server 2008 onwards to backeup data to a table, here we using DDL for creating table and DML for insertion at the definition level of SELECT INTO.
Is CRUD classified as DDL or DML or both.
CRUD being the operation Create Read Update and Delete
CRUD operations when applied to an SQL database map directly to DML statements. You have to bear in mind that the "C" in CRUD corresponds to the INSERT statement in SQL which creates an instance of something in a table. There is a CREATE statement in SQL that used for DDL operations but this is not the same sense of "Create" as in CRUD.
Create = INSERT
Read = SELECT
Update = UPDATE
Delete = DELETE
I'd say they're closer to Data Manipulation Language, because DDL is more about defining a schema where as DML is about working with objects within a defined schema. I propose the following map between DML and CRUD (using standard insert, create, update, delete methods from dml):
Create -- Insert
Read -- Select
Update -- Update
Delete -- Delete
However, I don't know if thinking of it as either DDL or DML will help you very much. CRUD isn't a language so much as a set of operations for handling objects you wish to expose via some interface.
I am newbie in SQL. I am reading about Triggers in SQL.I have got almost about Triggers. But in DML Triggers, we use FOR/AFTER keyword. I didn't get difference between FOR/AFTER and why we use FOR/AFTER keyword. I have already read on MSDN but didn't get the simple answer.
Can anyone explain me what is it?
Thanks in advance.
There is no difference between using FOR and AFTER.
I believe the original (pre 2000) syntax only used the FOR keyword. However, when INSTEAD OF triggers were introduced, the "FOR" keyword could seem quite confusing. "AFTER" more accurately conveys the type of trigger, and is more easily distinguished from "INSTEAD OF".
An INSTEAD OF trigger would be used if we wanted to transform what was inserted into the table, or prevent an insertion from taking place.
An AFTER trigger would more normally be used if we wanted to perform additional tasks, based on what has just occurred. For instance, you could have an "AFTER DELETE" trigger, that copied deleted rows into some kind of archive table. Basically, in an AFTER trigger, you more normally do still want the activity to occur.
From MSDN:
AFTER triggers are never executed if a constraint violation occurs; therefore, these triggers cannot be used for any processing that might prevent constraint violations.
And then:
You can request AFTER triggers by specifying either the AFTER or FOR keywords. Because the FOR keyword has the same effect as AFTER, DML triggers with the FOR keyword are also classified as AFTER triggers
It would seem there is no difference.
If I interpret your comments to the other answers correctly, you want to know why or when one uses the "FOR|AFTER" keywords.
It's simple: there are two kinds of triggers, the AFTER-trigger and the INSTEAD-OF-trigger.
The INSTEAD-OF-trigger for e.g. an insert action can be written as
create trigger myTrigger on myTable
INSTEAD OF insert
begin
(... code goes here ...)
end
and the AFTER-trigger can be written as either
create trigger myTrigger on myTable
AFTER insert
begin
(... code goes here ...)
end
or
create trigger myTrigger on myTable
FOR insert
begin
(... code goes here ...)
end
As Damien_The_Unbeliever mentions, the AFTER keyword is more readable than the FOR version, that is all.
They are the same. See this excerpt from BOL
"
FOR | AFTER
AFTER specifies that the DML trigger is fired only when all operations specified in the triggering SQL statement have executed successfully. All referential cascade actions and constraint checks also must succeed before this trigger fires.
AFTER is the default when FOR is the only keyword specified.
AFTER triggers cannot be defined on views.
"
According to what I observe, FOR is used in DDL trigger while AFTER is used in DML triggers. They have same way of working.