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I have a hierarchical table - it would suit a self referencing style, like this example I found.
CREATE TABLE OurStuff
(
StuffID INT NOT NULL PRIMARY KEY,
StuffSubID INT NULL,
StuffName VARCHAR(10) NOT NULL,
CONSTRAINT fk_StuffID FOREIGN KEY (StuffSubID)
REFERENCES OurStuff(StuffID)
)
I have a similar table - however when I try to replicate using Visual Studio Server Explorer, I get an error message:
"The columns in table 'abc' do not match an existing primary key or
unique constraint"
I can understand why, but I don't know why the above which is cited as an example would work and mine does not.
UPDATE: Here the link to the page in the example “How do I create a self-referencing foreign key?”
UPDATE:
I think you've got the relationship reversed. The ProductTypeIDcolumn should be on the left under the "Primary key table" and the ProductTypeParentID column should be on the right under the "Foreign key table."
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I have a question about designing a table for events.
Which one is better using a multi-column primary key, or using a sequential primary key with multi-column unique index?
Columns of this table are like this:
Generally in SQL Server, PRIMARY KEY is created as unique clustered index in the background.
So, it is good practice to keep clustered index key as:
Unique (avoids effort to add uniquifier to make the value unique)
Narrow (does not occupy lot of space)
Incremental (avoids fragmentation)
So, in your case , it is better to go for
Sequential Primary key & multi column unique index
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What are the good example columns for which I should never create an index? As per my understanding the clustered index should often be done on primary keys (default) as it represents base data as a whole. But on which columns I should never create a non-clustered index?
You cannot say for sure. The fact is: you cannot create an index on any column (or combination of columns) that has a max size of more than 900 bytes - so any columns like VARCHAR(1000) or VARCHAR(MAX) cannot be indexed.
Other than that - it reallly depends on what your system does! There's no magic rule what columns to index - or which to avoid.
In general: fewer indexes are better than too many. Most DB developers tend to over-index their databases - but as I said - this is really heavily dependent on the exact situation of your system - there's no simple, general rules to follow here.
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I am designing a database with a table to store vehicles and since the vehicle identification number is a 17 digit alphanumeric serial number my idea is to use it as the primary key, with a datatype of char(17).
Numerous other tables will then have the VIN as a foreign key.
A number of queries/searches will run with the VIN number as parameter since it's how we would like to track the vehicles as well as other data related to it.
The VIN number will never change, but I'm unsure if it would cause any serious performance degradation (or other complications I'm not aware of) since some queries will use joins and others not :/
By using the VIN as primary key I do not have to create a unique constraint / additional index - BUT it has to be char(17) a data type other than int for which primary keys are supposedly optimized...
What I'm also not 200% sure of is that every VIN number out there is the same length (very unlikely) but in that case how would using a varchar(17) affect the whole situation... if at all.
Thanks!
Just a personal opinion..
I always use int as a primary key. Primary key is in most cases always a clustered index. It's 4 bytes vs. 17 bytes and you can always put a non-clustered index on your VIN column. Keep things simple and clear. It's just my opinion though.
In my opinion, regarding performance, it indeed is not a good idea. It very much depends how many cars you will store in the database though. On the other hand, if your applications and queries use the VIN as parameter then it is the best option as the column is indexed and must be unique.
hope this helps
ps: akward seeing other people's suggestions on this topic!
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I am an SQL Rookie, I am trying to create a table but I have this error.
CREATE TABLE User
(
UserID int NOT NULL PRIMARY KEY,
UserName varchar(50) NOT NULL,
Email varchar(50) NOT NULL UNIQUE,
MembershipInfo varchar(50),
MembershipRank varchar(50),
CatID int,
CONSTRAINT CatID FOREIGN KEY(CatID) REFERENCES Category(CategoryID)
)
USERis a reserved word in Oracle and can't be used as a table name. The solution is to use another name.
USER is a reserved word. While it may be technically possible to force the database to let you create a table using a reserved word, it's a really bad idea. Just pick a new name - USERS is popular for this reason.
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Imagine I have a table with only 2 columns (FKs to other tables). I want to define "the primary key of this table is the combination of the 2 values".
What happens if I don't have a PK in this kind of table?
Without a UNIQUE constraint or unique index defined on the two columns, the table could have duplicate rows.
Also, a primary key is a clustered index by default: you would need to separately index the table for expected query performance.
Refer to another SO question and yet another SO question declared as a duplicate of it regarding the differences between primary key & unique constraints and unique indexes.