so I found this sql query in a project I am succeeding. This is the first time I encountering this clause/statement. I understand that this is to look if the table exist before creating one and that Object_ID is the table name that is to be created.
My questions are:
Does sysobject mean the database?
What is the Object property?
I know that it is not the columns inside the table to be created.
The columns are : dtb_color_id and description.
can someone explain this to me. please?
IF NOT EXISTS(SELECT * FROM SYSOBJECTS WHERE ID = OBJECT_ID('DTB_COLOR') AND OBJECTPROPERTY(ID,'ISUserTable') = 1)
BEGIN
.......some query I understand
END
sysobjects, OBJECTPROPERTY and OBJECT_ID are used in Microsoft SQL Server. They are part of the SQL Server DMVs and system functions/procedures used to query and manipulate the metadata.
sys.sysobjects is simply the list of all objects (tables, views, SPs, functions, etc) on the server in the active database. Please note, that sys.sysobjects is deprecated and is only available for backward compatibility. Use sys.objects instead
https://learn.microsoft.com/en-us/sql/relational-databases/system-dynamic-management-views/system-dynamic-management-views?view=sql-server-ver16
It has (as far as I know) no meaning in MySQL, unless somebody specifically created them.
You can also use INFORMATION_SCHEMA which is available in MySQL too (however slightly different in different RDBMS).
MSSQL INFORMATION_SCHEMA: https://learn.microsoft.com/en-us/sql/relational-databases/system-information-schema-views/system-information-schema-views-transact-sql?view=sql-server-ver16
MySQL INFORMATION_SCHEMA: https://dev.mysql.com/doc/refman/8.0/en/information-schema.html
SQL Server has no CREATE TABLE IF NOT EXISTS construct, a variation of the mentioned condition is commonly used to imitate that.
This is a way in SQL Server to check if a table exists in the active database and to perform actions according to the result, like creating the table.
OBJECTPROPERTY simply checks (in this case) if the table is a user created one.
https://learn.microsoft.com/en-us/sql/t-sql/functions/objectproperty-transact-sql?view=sql-server-ver16
I would remove the OBJECTPROPERTY condition in case the part you understand is a CREATE TABLE statement. You don't want to create a table which has a similar name to any system table/view, also you don't want to execute the CREATE TABLE if there is a VIEW with the same name (table creation will fail)
Yes sysobject means database.
The OBJECTPROPERTY() function returns information about schema-
scoped objects in the current database. Use this to check if an
object is a table, view, stored procedure, etc. You can also use
it to check if a table has a primary key, foreign key, foreign
key reference, etc.
For more details : https://learn.microsoft.com/en-us/sql/t-sql/functions/objectpropertyex-transact-sql?view=sql-server-ver16
In this scenario it is used to check whether it is user table or
not. The result of the ISUserTable property is 1 when it is user
table otherwise returns 0.
Here the following steps are followed:
First, it executes the select statement inside the IF Exists
If the select statement returns a value that condition is TRUE for IF Exists
It starts the code inside a begin statement
DTB_COLOR - May be a stored procedure
Related
I am using SQL Server. I found the following way to backup a database table:
-- Taking a backup
SELECT * INTO MY_BACKUP_DATABASE..CustomersTemporaryTable FROM Customers
I am trying to understand the .. in the syntax. From what I understand, the sentence means that Customers is the table that is going to be backed-up by placing it all of its content into the database called MY_BACKUP_DATABASE using CustomersTemporaryTable as the destination table. I assume when executing the sentence, CustomersTemporaryTable must already exist. Is my understanding of the sentence to take a backup correct?
Each MS SQL Table identifiers can have a name compound of three parts separates with a dot :
the database name
the SQL schema name (by default dbo)
the table, view or Table UDF name
Syntax :
db_name.schema_name.table_name
But it is not always necessary to specify the three parts.
Inside the current database, no need to specify the db_name. It's implicit...
By default every SQL user is associate with a specific default schema (frequently dbo too...).
So you can specify a table name with :
schema_name.table_name
...SQL Server will try to find the table into the current DB
db_name..table_name
...SQL Server will try to find the table into the specified DB and the default user schema
table_name
...SQL Server will try to find the table into the current DB and the
default user schema
To know with SQL schema is associated with your SQL user, use :
SELECT SCHEMA_NAME() AS DEFAULT_CURRENT_USER_SCHEMA
To know all the associations between SQL users and SQL schemas, do :
SELECT name AS USER_NAME, default_schema_name
FROM sys.database_principals
WHERE type_desc LIKE '%?_USER' ESCAPE '?'
First of all, understand that what you are doing is not "taking a backup", it is inserting data into a table from another table. If you have not created the destination table the syntax is like this:
Select *
INTO Destination_Table
FROM Source_Table
The destination table will be created automatically. This doesn't necessarily work so well if you will be inserting additional data that might be different lengths or data types, but for a one of select should work fine.
In my case there are different database versions (SQL Server). For example my table orders does have the column htmltext in version A, but in version B the column htmltext is missing.
Select [order_id], [order_date], [htmltext] from orders
I've got a huge (really huge statement), which is required to access to the column htmltext, if exists.
I know, I could do a if exists condition with two begin + end areas. But this would be very ugly, because my huge query would be twice in my whole SQL script (which contains a lot of huge statements).
Is there any possibility to select the column - but if the column not exists, it will be still ignored (or set to "null") instead of throwing an error (similar to the isnull() function)?
Thank you!
Create a View in both the versions..
In the version where the column htmltext exists then create it as
Create view vw_Table1
AS
select * from <your Table>
In the version where the htmlText does not exist then create it as
Create view vw_Table1
AS
select *,NULL as htmlText from <your Table>
Now, in your application code, you can safely use this view instead of the table itself and it behaves exactly as you requested.
First thing why a column would be missing? definitely its been deleted somewhere. if so, then the delete process must have updated/fixed the dependencies.
Instead fixing it after breaking, its better to do smart move by adopting some protocols before breaking anything.
IF Exists is a workaround that can help to keep queries running but its an overhead considering your huge database and queries
The "best" way to approach this is to check if the column exists in your database or not, and build your SQL query dynamically based on that information. I doubt if there is a more proper way to do this.
Checking if a column exists:
SELECT *
FROM sys.columns
WHERE Name = N'columnName'
AND Object_ID = Object_ID(N'tableName');
For more information: Dynamic SQL Statements in SQL Server
I struggled for a while with a bug, and then found out the reason for it in a database stored procedure code, which contained the old name of a database in a table name, whereas the current database name was already different. So, I'd like to ask:
Is there a situation in which using a database name as a part of a full table name (database name + schema name + table name) can be justified (provided we don't touch tables in other databases) or is it always a bad practice? How to correctly use a database name in sql scripts to keep code neutral to a specific database?
Code just for an illustration:
CREATE PROCEDURE [dbo].[MyProc]
AS
BEGIN
DELETE FROM [MyDatabase].[dbo].[MyTable]
END
No, you shouldn't use database names in a stored procedure unless you need to address two databases.
It causes exactly the kinds of bugs you're seeing. When the database name changes, all your SP code breaks, or continues working, but on the old database.
It does make sense if you are sending a SQL query to the database, but only if the application dynamically picks the database name to insert into the query.
My suggestion is that you do a full export of your database schema, and search for database names that are hardcoded and remove them.
It really depends on how your scripts are implemented.
Even if you don't refer to a table as
[MyDatabase].[dbo].[MyTable]
you will still need to refer to the database by:
USE [MyDatabase]
earlier in the script.
It is possible to mix trusted database tables in a single query. When someone do this,it is justified and mandatory to include database on table 'path'.
I don't found a reason out of this scenario if stored procedure and table is on the same database.
You can search all database name occurencies through database catalog in order to fix your development. For SQL Server 2005:
SELECT Name
FROM sys.procedures
WHERE OBJECT_DEFINITION(OBJECT_ID) LIKE '%databasename%'
GO
For SQL Server 2000:
SELECT DISTINCT so.name
FROM syscomments sc
INNER JOIN sysobjects so ON sc.id=so.id
WHERE sc.TEXT LIKE '%databasename%'
GO
I am migrating an MS Access application (which has linked tables to a MSSQL Server) to MySQL.
As a means to overcome some MSAccess table naming problems, I am seeking a solution to add a MySQL table alias that will point to an existing table in the MySQL database. Ideally I would like to create the alias 'dbo_customers' in mysql that would point to the customers table also in mysql.
To be clear I am not wanting to alias a table name inside a query like this:
SELECT * FROM customers AS dbo_customers
But rather I would like to be able issue the following query:
SELECT * FROM dbo_customers
and have it return data from the customers table.
Off the top of my head
CREATE VIEW dbo_customers AS
SELECT * FROM customers
Maybe not the best solution but should work as the view is updatable. Will definitely work for Read Only
You can create a View.
CREATE VIEW dbo_customers AS SELECT * FROM customers;
If that doesn't work for you, you could try creating a shadow-copy of the table, and use Triggers to keep the tables synced.
For example:
CREATE TABLE t1( id serial primary key, field varchar(255) not null );
CREATE TABLE dbo_t1( id serial primary key, field varchar(255) not null );
-- INSERT trigger
CREATE TRIGGER t1_dbo_insert AFTER INSERT ON t1
FOR EACH ROW BEGIN
INSERT INTO dbo_t1 SET field = NEW.field;
-- No need to specify the ID, it should stay in-sync
END
-- UPDATE trigger
CREATE TRIGGER t1_dbo_update AFTER UPDATE ON t1
FOR EACH ROW BEGIN
UPDATE dbo_t1 SET field = NEW.field WHERE id = NEW.id;
END
-- DELETE trigger
CREATE TRIGGER t1_dbo_delete AFTER DELETE ON t1
FOR EACH ROW BEGIN
DELETE FROM dbo_t1 WHERE id = OLD.id;
END
Not exactly an 'alias', and far from perfect. But it is an option if all else fails.
there is a simpler solution for MySQL via MERGE table engine:
imagine we have table named rus_vacancies and need its English equivalent
create table eng_vacancies select * from rus_vacancies;
delete from eng_vacancies;
alter table eng_vacancies ENGINE=MERGE;
alter table eng_vacancies UNION=(rus_vacancies);
now table rus_vacancies equals to table eng_vacancies for any read-write operations
one limitation - original table must have ENGINE=MyISAM (it can be easily done by "alter table rus_vacancies ENGINE=MyISAM")
You could create a view named dbo_customers which is backed by the customers table.
#OMG Ponies ponies said in a comment:
Why not rename the table?
...and it seems the obvious answer to me.
If you create an ODBC linked table for the MySQL table customers it will be called customers and then all you have to do is rename the table to dbo_customers. There is absolutely no need that I can see to create a view in MySQL for this purpose.
That said, I'd hate to have an Access app that was using SQL Server table names when the MySQL tables were not named the same thing -- that's just confusing and will lead to maintenance problems (i.e., it's simpler for the linked tables in the Access front end to have the same names as the MySQL tables, wherever possible). If I were in your position, I'd get a search and replace utility and replace all the SQL Server table names with the MySQL table names throughout the entire Access front end. You'd likely have to do it one table at a time, but in my opinion, the time it takes to do this now is going to be more than made up for in clarity going forward with development of the Access front end.
I always rename my "linked to SQL" tables in Access from
{dbo_NAME} to {NAME}.
The link creates the table name as {dbo_NAME} but access occasionally has problems with the dbo_ prefix.
Aliases would be nice, yet MySQL does NOT have such a feature.
One option that may serve your needs, besides creating a view, is to use the FEDERATED storage engine locally.
CREATE TABLE dbo_customers (
id INT(20) NOT NULL AUTO_INCREMENT,
name VARCHAR(32) NOT NULL DEFAULT '',
PRIMARY KEY (id),
)
ENGINE=FEDERATED
DEFAULT CHARSET=latin1
CONNECTION='mysql://fed_user#localhost:9306/federated/customers';
There are currently some limitations with the FEDERATED storage engine. Here are a couple especially important ones:
FEDERATED tables do not support transactions
FEDERATED tables do not work with the query cache
I'd like to mention a bad solution I explored (and abandoned), which was to use hardlinks on the .frm, .MYD and .MYI files corresponding to my table in /var/lib/mysql/{name_of_my_database}/.
It does, however, NOT work. For InnoDB tables, it simply cannot (even if you hardlink the .idb file) because tables are also referenced in ibdata1.
For MyISAM tables, it kind of works, except it doesn't because in memory, the tables are still distinct and thus do not share cache. So if you write a row to original_table, it won't immediately appear in aliased_table. You would have to flush tables first… which defeats the purpose and even causes data loss (if you insert a row in both the original and the alias before flushing, only one row is kept).
I thought my experiment was worth mentioning as a cautionary tale.
I am trying to insert data from one table to another with same structure,
select * into tbltable1 from tbltable1_Link
I am getting the following error message:
There is already an object named 'tbltable1' in the database.
The SELECT INTO statement creates a new table of the name you provide and populates it with the results of the SELECT statement.
I think you should be using INSERT INTO since the table already exists. If your purpose is in fact to populate a temporary table, then you should provide a table name that does not already exist in the database.
See MSDN for more information on this.
If you are confident that tbltable1 is not required, you can drop the table first.
You may also want to consider using temporary tables...
Select * into ##MyTemporaryTable FROM tblTable1_Link
You can then use the temporary table in this session. (Ending the session should drop the temporary table automatically, if I remember correctly. It's been a while since I've worked with SQL Server).