I am busy studying MySQL and I understand that update is used to update a record or row in a table. So what does alter do that is so different? Seems like they are the same.
Thanks, any help will be appreciated.
ALTER is a DDL (Data Definition Language) statement. Whereas UPDATE is a DML (Data Manipulation Language) statement. ALTER is used to update the structure of the table (add/remove field/index etc). Whereas UPDATE is used to update data.
The ALTER changes the table in the database, you can add or remove columns, etc. But it does not change data (except in the dropped or added columns of course).
While the UPDATE changes the rows in the table, and leaves the table unchanged.
ALTER is used to change things like table structures or stored procs, otherwise known as DDL statements.
ALTER table MyTable
ADD MyNewColumn VARCHAR(100)
OR
ALTER PROC dbo.MyStoredProc
Alter command is a data definition language
Update command is a data manipulation language
Alter example- table structure, table name, sp, functions
Update example-change database in a row or column etc
Alter command make changes in table structure.
Update command make changes with inside the table
Alter command is used to add, delete modify the attributes of the table in the database
Update command is used to update existing record in a database
Let's see in simple words...
Alter command we use for the modify the structure of the database, table(add, drop, modify)and it falls under DDL.
Update command we use for the modify the rows(records) of the table using where condition and its fall under DML.
Related
I want to alter column within the table to be NULL from NOT NULL.
The problem is that table has one constraint, one trigger and 4 indexes.
Can I somehow alter that column without dropping and recreating everything?
exec sys.sp_depends 'object_name'
List all dependencies and then drop them an recreate everything.
SQL Managements studio offer option with SCRIPT AS CREATE and you can save all views, triggers, tables as backup...
I have a table1 created in the prod environment. I have to add column to an existing table1.
Im using the function create of replace to create the table with the suitable ddl.
To add the column shall i add it manually on the prod machine and change the ddl.
Or changing only the ddl will do it?
Thank you
As far as i know there is no such thing as a create or replace table syntax. that should only work for functions or views.
Use the ALTER TABLE Table1 ADD NewColumn {DATA_TYPE} command.
The way you phrased your question makes me think you use a 3rd party database version control like liquibase. If that is the case leave the initial create DDL alone and just add it as a separate change. If you change the initial ddl it will only affect new databases.
HI I am generating schema script for a database, but when i finish creating it and and look at the script it gives create table statement for a table but not including all column in it also it generates alter table add column statement for the same tables but for missing columns which are left in create table statement.
see the attached screenshot.
Assuming the question is "why does it not create all of the columns in the CREATE TABLE statement" ...
You'll notice that between the CREATE TABLE and the ALTER statements, the value for SET ANSI_PADDING is altered. As the documentation notes, the setting's value is taken into account at the point in time when a column is created.
There's no way to override this setting in-line with the declaration of a column.
Since your table apparently contains a mixture of columns, some of which are defined with the setting ON and others with it defined OFF, there's no way to write a single CREATE TABLE statement that creates all of the columns in one go.
We are facing problem in the alter table SQL statement. Some time we update our database at client side and the alter table sql taking very much time. I like to know, how alter works? Does alter statement performance correlated to that table data? Means, if table have large data then alter will take much time.
There is also problem with the Oracle 11G R2. Is there any changes which need to incorporate to our code? Our code is very old and working fine till now?
There could be several reasons for this:
If the table is locked by another
query/resource. It would wait for the
lock to be released and then execute
the update...
If the table contains many rows and you have added a new column in the table with a default value, it would execute an update query for whole table after altering the table to update all the existing records with the default value...
If for example you add a new column with a default value in a large table, then it will take time depending the size of the table.
Is there any way how to update automatically mysql trigger(s) structure, every time when I alter a table?
No - any alteration that impacts the trigger requires you to update separately from the ALTER TABLE statement.