T-SQL Add Column In Specific Order_问答_开发者

Im a bit new to T-SQL, Coming from a MySQL background Im still adapting to the different nuances in the syntax.

Im looking to add a new column AFTER a specific one. I've found out that AFTER is a valid keyword but I don't think it's the right one for the job.

ALTER TABLE [dbo].[InvStockStatus]
ADD [Abbreviation] [nvarchar](32) DEFAULT '' NOT NULL ;

This is my 开发者_如何转开发current query, which works well, except it adds the field at the end of the Table, Id prefer to add it after [Name]. What's the syntax im looking for to represent this?

You can't do it like that

for example if you have a table like this

create table TestTable(id1 int,id3 int)

and you want to add another column id2 between id1 and id3 then here is what SQL Server does behind the scene if you use the designer

BEGIN TRANSACTION
SET QUOTED_IDENTIFIER ON
SET ARITHABORT ON
SET NUMERIC_ROUNDABORT OFF
SET CONCAT_NULL_YIELDS_NULL ON
SET ANSI_NULLS ON
SET ANSI_PADDING ON
SET ANSI_WARNINGS ON
COMMIT
BEGIN TRANSACTION
GO
CREATE TABLE dbo.Tmp_TestTable
    (
    id1 int NULL,
    id2 int NULL,
    id3 int NULL
    )  ON [PRIMARY]
GO
ALTER TABLE dbo.Tmp_TestTable SET (LOCK_ESCALATION = TABLE)
GO
IF EXISTS(SELECT * FROM dbo.TestTable)
     EXEC('INSERT INTO dbo.Tmp_TestTable (id1, id3)
        SELECT id1, id3 FROM dbo.TestTable WITH (HOLDLOCK TABLOCKX)')
GO
DROP TABLE dbo.TestTable
GO
EXECUTE sp_rename N'dbo.Tmp_TestTable', N'TestTable', 'OBJECT' 
GO
COMMIT

As you can see if you have a lot of data this can be problematic, why does it matter where the column is located? just use

select col1,col2,col3 from table

The sequence of columns is really irrelevant in a strict (functional) sense, in any RDBMS - it's just a "nicety" to have for documentation or humans to look at.

SQL Server doesn't support any T-SQL commands to order the columns in any way. So there is no syntax in T-SQL to accomplish this.

The only way to change that is to use the visual table designer in SSMS, which really recreates the whole table from scratch, when you move around columns or insert columns in the middle of a table.

While from a functional database perspective it is correct that the tuples can appear in any order.

However a database does not exist inside a vacuum. There is always a human that will want to read the table schema (dbas, devs) and get there head around it and maintain or write queries against it.

In the past, I've used conventions for a table's columns such as ordering a table with

primary key(s) first
then foreign keys
then frequently used columns
then other columns
and lastly audit related columns

and these help when scanning a table. Unfortunately, it appears you have to jump through hoops to maintain any order, so now I have to question whether it's worth having and maintaining these conventions. My new rule is just add it to the end.

If you are really worried about order from a readability perspective, you should create your own 'readability' views (perhaps in a different schema) in any order you feel like. You could have multiple views of the same table (one for just the core columns and another including stuff that usually isn't relevant).

It would be nice to be able to be able to re-order columns in SQL Server database diagrams (as a display thing only), but this isn't possible.

You should always add fields only at the end. You should select fields in the order you want, but never restructure an existing table to add a column inthe middle. This is likely to break some things (where people did dumb things like select * or inserts without specifying the columns granted people shouldn't do those things, but they do).

Recreating the table can be a long time-consuming process for no gain whatsoever and can cause lots of user complaints and lockups while it is going on.

The schema comparison tools I have seen will create a new table with the desired ordering and then copy the data from the old table to the new one (with some renaming magic to make the new one resemble the old). Given how akward this approach is, I figure there isn't a T-SQL statement to add a new column in a specific place.

This is a safe work around without using temp table. After you add the column towards the end, simply go to SQL Sever Management Studio. Click on the table, select -> Design (or Modify) and drag the last column to where ever position you want.

This way you do not have to worry about loosing data and indexes. The only other option is to recreate the table which can be problematic if you have large data.

This answer is for helping other people and not intended to be accepted as answer.

Technically, or perhaps I should say, academically, the order in which columns are added to a table, or the order in which they are stored in the database's internal storage model, should not be of any concern to you. You can simply list the columns in the Select clause of your SQL queries to control the order that columns or computed expressions appear in the output of any query you run. Internally the database is free to store the actual data any way it sees fit to optimize storage, and or help align data elements with disk and/or memory boundaries.

A fair amount of time has passed since this question was posted but it's worth noting that while the underlying raw SQL code to place columns in a specific order hasn't changed the process of generating scripts to do it is taken care of if you choose to use SQL Server Data Tools within Visual Studio to manage your database. The database you deploy to will always have the columns in the order that you specify in your project.