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A table with a filed nvarchar(50) – how to minimize the resourses for customers that use this filed with NULL value ?

Posted on 2014-01-06
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Last Modified: 2014-01-07
Dear experts,

 I’m using SQL Server 2008, may be will go to 2012 in near future and I have a question.

I have a table with 10 filleds, that will go to some millions records. In that table I should add a nvarchar(50) which will be used only for some customers, for others this Keyword nvarchar(50) filed will be empty.

 So my question is – for the customers that will NOT use at all this Keyword field, this will increase the size of DB or not?  What exactly type I should use and at all, what should I do to minimize the recourses used for the customer with Keywords = null ??? I prefer not to make different logic …
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Question by:dvplayltd
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by:
Guy Hengel [angelIII / a3] earned 2000 total points
ID: 39761433
there shall be no increase for the existing rows.
for NULLable fields, if the field value IS null, a single "bit" in the data will indicate the field value is null. for existing records, this will not change any data, as that bismask will be in a existing byte for the already existing columns.

for the rest, you data type is ok, you should only make sure that the field accepts NULL (is nullable) which is usually the default
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Expert Comment

by:Surrano
ID: 39761444
In such cases I'd recommend putting this extra field into a "detail" table which is in 1:1 relationship with the "master" table. Joins with appropriate indexing should be piece of cake; the only tradeoff would be creating a unique index on the master table if doesn't exist yet but I assume it does for a table with millions of rows.
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Expert Comment

by:HuaMinChen
ID: 39761448
1. You can use isnull(column1,'') to select also those records having null value on this column;
2. Use Insert/Update triggers to enforce that the users have to put values on it, if you want to have value on it

Yes, correct database size is depending on whether there are values on the columns or not.
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Author Closing Comment

by:dvplayltd
ID: 39761518
10x for your answer. One more clarify - if I set the value to empty string - this is the same as set to NULL or not ?
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Expert Comment

by:Guy Hengel [angelIII / a3]
ID: 39761522
in sql server, empty string is NOT the same as NULL
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Expert Comment

by:Surrano
ID: 39761583
I think dvplayltd's question is about space usage; i.e. does an empty string occupy more space than a null in case of an nvarchar50 field? My gut feeling says yes, empty string must occupy more space, but I'm not familiar enough with SQL Server 20xx to tell for sure.
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Expert Comment

by:Guy Hengel [angelIII / a3]
ID: 39761686
yes, it does. because NULL will only be 1 bit in the "null bitmask", while empty string will have the information of how long the string actually is in a "header" byte/double-byte.
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