i have a table T with following columns
col1 col2 col3 col4
1 1 1 1
1 1 1 2
1 1 2 1
1 1 2 1
if i set a column col2,col3,col4 as unique. how does the unique works ? will it take uniqueness of combination of each column?
See here: http://www.w3schools.com/sql/sql_unique.asp
The syntax for setting multiple columns as unique is different from that of setting one column unique. If you have multiple columns as unique it is the set that is viewed for uniqueness.
Yes, the "unique-ness" is a result of all columns involved in the constraint. See SO Question
You can easily write yourself a table and test how it handles INSERTs
I'm not entirely sure, but I think the unique attribute has to do with indexing the table. Whichever column you set as unique, that column should be the one you call on to find a certain row. For example in a call like
UPDATE table_name SET column_name = some_value WHERE ID = some_number
the ID column should be set to unique, though I don't know whether not doing so would actually stop you from finding a specific row.
Related
I have a column that store user input text field from a frontend website. User can input any kind of text in it, but they will also put in a specific alphabets combination to represent a job type - for example 'dri'. As an example:
Row 1: P49384; Open vehicle bonnet-BO-dri 22/10
Row 2: P93818; Vehicle exhaust-BO 10/20
Row 3: P1933; battery dri-pu-103/2
Row 4: P3193; screwdriver-pu 423
Row 5: X939; seats bo
Row 6: P9381-vehicle-pu-bo dri
In this case, I will like to filter only rows that contain dri. From the example, you can see the text can be in any order (user behaviour, they will key whatever they like without following any kind of format). But the constant is that for a particular job type, they will put in dri.
I know that I can simply use LIKE in SQL Server to get these rows. Unfortunately, row 4 is included inside when I use this operator. This is because screwdriver contains dri.
Is there any way in SQL Server I can do to strictly only obtain rows that has dri job type, while excluding words like screwdriver?
I tried to use PATINDEX but it failed too - PATINDEX('%[d][r][i]%', column) > 0
Thanks in advance.
Your data is the problem here. Unfortunately even for denormalised data it doesn't appear to have a reliable/defined format, making parsing your data in a language like T-SQL next to impossible. What problems are there? Based on the original sample data, at a glance the following problems exist:
The first data value's delimiter isn't consistent. Rows 1-5 use a semicolon (;), but row 6 uses a hyphen (-)
The last data value's delimiter isn't consistent. Row 1, 2 & 4 use a space ( ), but row 3 uses a hyphen (-).
Internal data doesn't use a consistent delimiter. For example:
Row 1 has a the value Open vehicle bonnet-BO-dri, which appears to be the values Open vehicle bonnet, BO and dri; so the hyphen(-) is the delimiter.
Row 5 has seats bo, which appears to be the values seats and bo, so uses a space ( ) as a delimiter.
The fact that row 6 has vehicle as its own value (vehicle-pu-bo-dri), however, implies that Open vehicle bonnet and Vehicle Exhaust (on rows 1 and 2 respectively) could actually be the values Open, vehicle, & bonnet and Vehicle & Exhaust respectively.
Honestly, the solution is to fix your design. As such, your tables should likely look something like this:
CREATE TABLE dbo.Job (JobID varchar(6) CONSTRAINT PK_JobID PRIMARY KEY NONCLUSTERED, --NONCLUSTERED Because it's not always ascending
YourNumericalLikeValue varchar(5) NULL); --Obviously use a better name
CREATE TABLE dbo.JobTypeCompleted(JobTypeID int IDENTITY (1,1) CONSTRAINT PK_JobTypeID PRIMARY KEY CLUSTERED,
JobID varchar(6) NOT NULL CONSTRAINT FK_JobType_Job FOREIGN KEY REFERENCES dbo.Job (JobID),
JobType varchar(30) NOT NULL); --Must likely this'll actually be a foreign key to an actual job type table
GO
Then, for a couple of your rows, the data would be inserted like so:
INSERT INTO dbo.Job (JobID, YourNumericalLikeValue)
VALUES('P49384','22/10'),
('P9381',NULL);
GO
INSERT INTO dbo.JobTypeCompleted(JobID,JobType)
VALUES('P49384','Open vehicle bonnet'),
('P49384','BO'),
('P49384','dri'),
('P9381','vehicle'),
('P9381','pu'),
('P9381','bo'),
('P9381','dri');
Then you can easily get the jobs you want with a simple query:
SELECT J.JobID,
J.YourNumericalLikeValue
FROM dbo.Job J
WHERE EXISTS (SELECT 1
FROM dbo.JobTypeCompleted JTC
WHERE JTC.JobID = J.JobID
AND JTC.JobType = 'dri');
You can apply like operator in your query as column_name like '%-dri'. It means find out records that end with "-dri"
I have a table of +15 million rows and 36 columns, there are two rows of data for every object to which the table refers. I need to:
Move one Column 0 down one space so that the useful information from that column appears in the row below.
Here is a sample of the data with less columns:
Table name = ekd0310
I want to shift Column 0 down 1
Column 0 Column 1 Column 2 Column 3
B02100AA.CZE
B02100AA.CZF I MIGA0027 SUBDIREC.019
B02100AA.CZG
B02100AA.CZH I MIGA0027 SUBDIREC.019
B02100AA.CZI
B02100AA.CZJ I MIGA0027 SUBDIREC.019
B02100AA.CZK '
THe function that you are looking for is probably lead(). You can use this if you assume that there is a column that specifies the ordering. An example:
select e.*, lead(col) over (order by id) as nextcol
from ekd0310 e;
Although this is an ANSI standard function, not all databases support it (yet). You can do something similar with correlated subqueries. Similarly, the above returns the information, but it is possible to do this as an update as well.
I have this simple table :
Table Users
userId | name
---------------------
1 'a1'
2 'a2'
3 'a3'
4 'a4'
5 'a5'
Table Cities
cityId | name
---------------------
1 'c1'
2 'c2'
3 'c3'
4 'c4'
5 'c5'
Each user is can be in more than one city. :
So the mapping table is :
userId | CityId
------------------------------------
1 4
1 4
1 4
2 5
5 6
Table users is heavily scanned by name .
Question :
For the mapping table I have no issues. both columns together are primary/clustered index.
But i'm struggling with myself about the first 2 tables :
I think that Users should have userId column as primary key. why ? because it is used throug the join to the mapping table.
but I also need clustered index on the name column cause this table is heavily scanned by name.
(leave aside the unique problem. lets say all columns are unique)
What is the best practice decision for this case ?
The best decision depends on how exactly you use the data returned by a query.
A clustered index means that the data in the page files are ordered based on this index.
A regular index will have it's own page files to order the index and a pointer to the physical row.
Thus a clustered index will serve better for theses queries that return a range of value instead of unique rows.
So, unless you do a lot of queries with like operations on the Name column, you would be better to keep your clustered index on the ID column, for this index will be constantly scanned and used to return recordsets to support your join operations.
How can I show the number of rows in a table in a way that when a new record is added the number representing the row goes higher and when a record is deleted the number gets updated accordingly?
To be more clear,suppose I have a simple table like this :
ID int (primary key) Name varchar(5)
The ID is set to get incremented by itself (using identity specification) so it can't represent the number of row(record) since if I have for example 3 records as:
ID NAME
1 Alex
2 Scott
3 Sara
and I delete Alex and Scott and add a new record it will be:
3 Sara
4 Mina
So basically I'm looking for a sql-side solution for doing this so that I don't change anything else in the source code in multiple places.
I tried to write something to get the job done but it failes. Here it is :
SELECT COUNT(*) AS [row number],Name
FROM dbo.Test
GROUP BY ID, Name
HAVING (ID = ID)
This shows as:
row number Name
1 Alex
1 Scott
1 Sara
while I want it to get shown as:
row number Name
1 Alex
2 Scott
3 Sara
If you just want the number against the rows while selecting the data and not in the database then you can use this
select row_number() over(order by id) from dbo.Test
This will give the row number n for nth row.
Try
SELECT id, name, ROW_NUMBER() OVER (ORDER BY id) AS RowNumber
FROM MyTable
What you want is called an auto increment.
For SQL-Server this is achieved by adding the IDENTITY(1,1) attribute to the table definition.
Other RDBMS use a different syntax. Firebird for example has generators, which do the counting. In a BEFORE-INSERT trigger you would assign the ID-field to the current value of the generator (which will be increased automatically).
I had this exact problem a while ago, but I was using SQL Server 2000, so although row number() is the best solution, in SQL Server 2000, this isn't available. A workaround for this is to create a temporary table, insert all the values with auto increment, and replace the current table with the new table in T-SQL.
Let's assume I have following table in the database:
Id ProductId ColorId IsDeleted
1 1 1 1
2 1 1 0
3 2 3 0
I want to make ProductId and ColorId columns unique but only for those rows where IsDeleted = 0. How do I can achieve this requirement?
I know, I can create a constraint which will call a stored function. And stored function will try to find an entry with the same values. But I think it is to complex way. May be there is a better decision...
In SQL Server 2008 and newer, you can take advantage of filtered indices to achieve this:
CREATE NONCLUSTERED UNIQUE INDEX ProductColor
ON dbo.YourTable(ProductID, ColorID)
WHERE IsDeleted = 0;
See the Filtered Index Design Guidelines for some more background and best practices.