I have a table ProductAmount with columns
Id [BIGINT]
Amount [FLOAT]
now when I pass value from my form to table it gets stored in format 2.46237846387469E+15 whereas actual value was 2462378463874687. Any ideas why this value is being converted and how to stop this?
It is not being converted. That is what the floating point representation is. What you are seeing is the scientific/exponential format.
I am guessing that you don't want to store the data that way. You can alter the column to use a fixed format representation:
alter table ProductAmount alter amount decimal(20, 0);
This assumes that you do not want any decimal places. You can read more about decimal formats in the documentation.
I would strongly discourage you from using float unless:
You have a real floating point number (say an expected value from a statistical calculation).
You have a wide range of values (say, 0.00000001 to 1,000,000,000,000,000).
You only need a fixed number of digits of precision over a wide range of magnitudes.
Floating point numbers are generally not needed for general-purpose and business applications.
The value gets stored in a binary format, because this is what you specified by requesting FLOAT as the data type for the column.
The value that you store in the field is represented exactly, because 64-bit FLOAT uses 52 bits to represent the mantissa*. Even though you see 2.46237846387469E+15 when selecting the value back, it's only the presentation that is slightly off: the actual value stored in the database matches the data that you inserted.
But i want to store 2462378463874687 as a value in my db
You are already doing it. This is the exact value stored in the field. You just cannot see it, because querying tool of SQL Management Studio formats it using scientific notation. When you do any computations on the value, or read it back into a double field in your program, you will get back 2462378463874687.
If you would like to see the exact number in your select query in SQL Management Studio, use CONVERT:
CONVERT (VARCHAR(50), float_field, 128) -- See note below
Note 1: 128 is a deprecated format. It will work with SQL Server-2008, which is one of the tags of your question, but in versions of SQL Server 2016 and above you need to use 3 instead.
Note 2: Since the name of the column is Amount, good chances are that you are looking for a different data type. Look into decimal data types, which provide a much better fit for representing monetary amounts.
* 2462378463874687 is right on the border for exact representation, because it uses all 52 bits of mantissa.
I would like to change the manner in which the mileage is represented in the database. For example, right now the mileage is represented as 080+0.348; this would mean that this particular feature is at mileage point 80.348 along the roadway corridor. I would like to have the data represented in the database in the latter form, 80.348 and so on. This would save me from having to export the dataset to excel for the conversion. Is this even possible? The name of the column is NRLG_MILEPOINT.
Much appreciated.
One thing you could try is to pick the string value apart into its component pieces and then recombine them as a number. If your data is in a table called TEST you might do something like the following:
select miles, fraction,
nvl(to_number(miles), 0) + nvl(to_number(fraction), 0) as milepoint
from (select regexp_substr(nrlg_milepoint, '[0-9]*') as miles,
regexp_substr(nrlg_milepoint, '[+-][0-9.]*') as fraction
from test);
SQLFiddle here.
Share and enjoy.
Using the answer provided above, I was able to expand it to get exactly the answer i needed. Thanks a ton to everyone who helped! Here is the query i ended up with:
select distinct nrlg_dept_route,corridor_code_rb,nrlg_county,next_county,
nvl(to_number(miles), 0) + nvl(to_number(fraction), 0) as milepoint
from (select regexp_substr(nrlg_milepoint, '[0-9]*') as miles,
nrlg_milepoint as nrlg_mile_point
nrlg_dept_route as nrlg_dept_route,
nrlg_county as nrlg_county,
next_county as next_county,
corridor_code_rb as corridor_code_rb,
corridor_code as corridor_code,
regexp_substr(nrlg_milepoint, '[+-][0-9.]*') as fraction
from corridor_county_intersect,south_van_data_view)
where nrlg_dept_route = corridor_code
order by 1,5;
There are a variety of ways to do this. Which one depends on your situation, how the data needs to be stored, and how it is being interacted with. Some of these options include:
Changing the datatype.
This option would potentially require you to change how the data is being stored currently. The conversion of the data would have to be done by whatever is writing the data to the schema currently.
Creating another column that stores the data in the correct format.
If you have an existing means of storing the data that would be broken by changing the datatype of NRLG_MILEPOINT and/or you have a requirement to store the data in that format; you could optionally add another column... say NRLG_MILEAGE_DISPLAY that is of a datatype number perhaps, and store the data there. You could make a trigger that updates/inserts NRLG_MILEAGE_DISPLAY appropriately, based on the data in NRLG_MILEPOINT.
If you are just wanting the data to be displayed differently in your select statement, you can convert the datatype in your SQL statement. Specifically how you would do this depends on the current datatype of NRLG_MILEPOINT.
Assuming that varchar2 is the type, based on the comments, here is an SQLFIDDLE link displaying a crude example of option 3. Your usage of this may vary depending on the actual datatype of NRLG_MILEPOINT. Regardless of its datatype... I am sure there is a means of converting how it is displayed in your query. You could take this further and create a view if you needed to. As an inline view or as a stored view, you can then use the converted value for doing your join later.
I need to store Medicare APC codes. I believe the format requires 4 numbers. Leading zeros are relevant. Is there any way to store this data type with verification? How should I store this data (varchar(4), int)?
This kind of issue, storing zero leading numbers that need to be treated as Numeric values on some scenarios (i.e. sorting) and as textual values in others (i.e. addresses) is always a pain and there is no one answer that is best for all users. At my company we have a database that stores numbers as text for codes (not Medicare APC codes) and we must pad them with zero’s so they will sort properly when used in an order operation.
Do not use a numeric data type for this because the item is not a true number but textual data that uses numeric characters. You will not be performing any calculations or aggregates on the codes and so the only benefit to storing them as a number would be to ensure proper sorting of the codes and that can be done with the code stored as text by padding it with zeros where needed. If you sue a numeric data type then any time the code is combined with other textual values you will have to explicitly convert it to CHAR/VARCHAR or let SQL Server do it since implicit conversions should always be avoided that means a lot of extra work for you and the query processor any time the code is used.
Assuming you decide to go with a textual data type the question then is should you use VARCHAR or CHAR and while many who have posted say VARCHAR I would suggest you go with CHAR set to a length of 4. WHY?
The VARCHAR data type is for textual data in which the size (the length or number of characters) is unknown in advance. For this Medicare code we know the length will always be at least 4 and possibly no more than 4 for the foreseeable future. SQL Server handles the storage of the data differently between CHAR and VARCHAR. SQL Server’s BOL (Books On Line) says :
Use CHAR when the size of the column data entries are consistent
Use VARCHAR when the size of the column data varies considerably.
I can’t say for certain this is true for SQL Server 2008 and up but for earlier versions, the use of a VARCHAR data type carries an extra overhead of 1 byte per row of data per column in a table that has a VARCHAR data type. If the data stored is always the same size and in your scenario it sounds like it is then this extra byte is a waste.
In the end it’s up to you as to whether you like CHAR or VARCHAR better but definitely don’t use a numeric data type to store a fixed length code.
That's not numeric data; it's textual data that happens to contain digits.
Use a VARCHAR.
I agree, using
CHAR(4)
for the check constraint use
check( APC_ODE LIKE '[0-9][0-9][0-9][0-9]' )
This will force a 4 digit number only to be accepted...
varchar(4)
optionally, you can still add a check constraint to ensure the data is numeric with leading zeros. This example will throw exceptions in Oracle. In other RDBMS, you could use regular expression checks:
alter table X add constraint C
check (cast(APC_CODE as int) = cast(APC_CODE as int))
If you are certain that the APC codes will always be numeric (that is if it wouldn't change in the near future), a better way would be to leave the database column as is, and handle the formatting (to include leading zeros) at places where you use this field values.
If you need leading 0s, then you must use a varchar or other string data type.
There are ways to format the output for leading 0s without compromising your actual data.
See this blog entry for an easy method.
CHAR(4) seems more appropriate to me (if I understood you right, and the code is always 4 digits).
What you want to use is a VARCHAR data type with a CHECK constraint, using LIKE with a pattern to check for numeric values.
in TSQL
check( isnumeric(APC_ODE) = 1)
I realize that CHAR is recommended if all my values are fixed-width. But, so what? Why not just pick VARCHAR for all text fields just to be safe.
The general rule is to pick CHAR if all rows will have close to the same length. Pick VARCHAR (or NVARCHAR) when the length varies significantly. CHAR may also be a bit faster because all the rows are of the same length.
It varies by DB implementation, but generally, VARCHAR (or NVARCHAR) uses one or two more bytes of storage (for length or termination) in addition to the actual data. So (assuming you are using a one-byte character set) storing the word "FooBar"
CHAR(6) = 6 bytes (no overhead)
VARCHAR(100) = 8 bytes (2 bytes of overhead)
CHAR(10) = 10 bytes (4 bytes of waste)
The bottom line is CHAR can be faster and more space-efficient for data of relatively the same length (within two characters length difference).
Note: Microsoft SQL has 2 bytes of overhead for a VARCHAR. This may vary from DB to DB, but generally, there is at least 1 byte of overhead needed to indicate length or EOL on a VARCHAR.
As was pointed out by Gaven in the comments: Things change when it comes to multi-byte characters sets, and is a is case where VARCHAR becomes a much better choice.
A note about the declared length of the VARCHAR: Because it stores the length of the actual content, then you don't waste unused length. So storing 6 characters in VARCHAR(6), VARCHAR(100), or VARCHAR(MAX) uses the same amount of storage. Read more about the differences when using VARCHAR(MAX). You declare a maximum size in VARCHAR to limit how much is stored.
In the comments AlwaysLearning pointed out that the Microsoft Transact-SQL docs seem to say the opposite. I would suggest that is an error or at least the docs are unclear.
If you're working with me and you're working with Oracle, I would probably make you use varchar in almost every circumstance. The assumption that char uses less processing power than varchar may be true...for now...but database engines get better over time and this sort of general rule has the making of a future "myth".
Another thing: I have never seen a performance problem because someone decided to go with varchar. You will make much better use of your time writing good code (fewer calls to the database) and efficient SQL (how do indexes work, how does the optimizer make decisions, why is exists faster than in usually...).
Final thought: I have seen all sorts of problems with use of CHAR, people looking for '' when they should be looking for ' ', or people looking for 'FOO' when they should be looking for 'FOO (bunch of spaces here)', or people not trimming the trailing blanks, or bugs with Powerbuilder adding up to 2000 blanks to the value it returns from an Oracle procedure.
In addition to performance benefits, CHAR can be used to indicate that all values should be the same length, e.g., a column for U.S. state abbreviations.
Char is a little bit faster, so if you have a column that you KNOW will be a certain length, use char. For example, storing (M)ale/(F)emale/(U)nknown for gender, or 2 characters for a US state.
Does NChar or Char perform better that their var alternatives?
Great question. The simple answer is yes in certain situations. Let's see if this can be explained.
Obviously we all know that if I create a table with a column of varchar(255) (let's call this column myColumn) and insert a million rows but put only a few characters into myColumn for each row, the table will be much smaller (overall number of data pages needed by the storage engine) than if I had created myColumn as char(255). Anytime I do an operation (DML) on that table and request alot of rows, it will be faster when myColumn is varchar because I don't have to move around all those "extra" spaces at the end. Move, as in when SQL Server does internal sorts such as during a distinct or union operation, or if it chooses a merge during it's query plan, etc. Move could also mean the time it takes to get the data from the server to my local pc or to another computer or wherever it is going to be consumed.
But there is some overhead in using varchar. SQL Server has to use a two byte indicator (overhead) to, on each row, to know how many bytes that particular row's myColumn has in it. It's not the extra 2 bytes that presents the problem, it's the having to "decode" the length of the data in myColumn on every row.
In my experiences it makes the most sense to use char instead of varchar on columns that will be joined to in queries. For example the primary key of a table, or some other column that will be indexed. CustomerNumber on a demographic table, or CodeID on a decode table, or perhaps OrderNumber on an order table. By using char, the query engine can more quickly perform the join because it can do straight pointer arithmetic (deterministically) rather than having to move it's pointers a variable amount of bytes as it reads the pages. I know I might have lost you on that last sentence. Joins in SQL Server are based around the idea of "predicates." A predicate is a condition. For example myColumn = 1, or OrderNumber < 500.
So if SQL Server is performing a DML statement, and the predicates, or "keys" being joined on are a fixed length (char), the query engine doesn't have to do as much work to match rows from one table to rows from another table. It won't have to find out how long the data is in the row and then walk down the string to find the end. All that takes time.
Now bear in mind this can easily be poorly implemented. I have seen char used for primary key fields in online systems. The width must be kept small i.e. char(15) or something reasonable. And it works best in online systems because you are usually only retrieving or upserting a small number of rows, so having to "rtrim" those trailing spaces you'll get in the result set is a trivial task as opposed to having to join millions of rows from one table to millions of rows on another table.
Another reason CHAR makes sense over varchar on online systems is that it reduces page splits. By using char, you are essentially "reserving" (and wasting) that space so if a user comes along later and puts more data into that column SQL has already allocated space for it and in it goes.
Another reason to use CHAR is similar to the second reason. If a programmer or user does a "batch" update to millions of rows, adding some sentence to a note field for example, you won't get a call from your DBA in the middle of the night wondering why their drives are full. In other words, it leads to more predictable growth of the size of a database.
So those are 3 ways an online (OLTP) system can benefit from char over varchar. I hardly ever use char in a warehouse/analysis/OLAP scenario because usually you have SO much data that all those char columns can add up to lots of wasted space.
Keep in mind that char can make your database much larger but most backup tools have data compression so your backups tend to be about the same size as if you had used varchar. For example LiteSpeed or RedGate SQL Backup.
Another use is in views created for exporting data to a fixed width file. Let's say I have to export some data to a flat file to be read by a mainframe. It is fixed width (not delimited). I like to store the data in my "staging" table as varchar (thus consuming less space on my database) and then use a view to CAST everything to it's char equivalent, with the length corresponding to the width of the fixed width for that column. For example:
create table tblStagingTable (
pkID BIGINT (IDENTITY,1,1),
CustomerFirstName varchar(30),
CustomerLastName varchar(30),
CustomerCityStateZip varchar(100),
CustomerCurrentBalance money )
insert into tblStagingTable
(CustomerFirstName,CustomerLastName, CustomerCityStateZip) ('Joe','Blow','123 Main St Washington, MD 12345', 123.45)
create view vwStagingTable AS
SELECT CustomerFirstName = CAST(CustomerFirstName as CHAR(30)),
CustomerLastName = CAST(CustomerLastName as CHAR(30)),
CustomerCityStateZip = CAST(CustomerCityStateZip as CHAR(100)),
CustomerCurrentBalance = CAST(CAST(CustomerCurrentBalance as NUMERIC(9,2)) AS CHAR(10))
SELECT * from vwStagingTable
This is cool because internally my data takes up less space because it's using varchar. But when I use DTS or SSIS or even just a cut and paste from SSMS to Notepad, I can use the view and get the right number of trailing spaces. In DTS we used to have a feature called, damn I forget I think it was called "suggest columns" or something. In SSIS you can't do that anymore, you have to tediously define the flat file connection manager. But since you have your view setup, SSIS can know the width of each column and it can save alot of time when building your data flow tasks.
So bottom line... use varchar. There are a very small number of reasons to use char and it's only for performance reasons. If you have a system with hundrends of millions of rows you will see a noticeable difference if the predicates are deterministic (char) but for most systems using char is simply wasting space.
Hope that helps.
Jeff
There are performance benefits, but here is one that has not been mentioned: row migration. With char, you reserve the entire space in advance.So let's says you have a char(1000), and you store 10 characters, you will use up all 1000 charaters of space. In a varchar2(1000), you will only use 10 characters. The problem comes when you modify the data. Let's say you update the column to now contain 900 characters. It is possible that the space to expand the varchar is not available in the current block. In that case, the DB engine must migrate the row to another block, and make a pointer in the original block to the new row in the new block. To read this data, the DB engine will now have to read 2 blocks.
No one can equivocally say that varchar or char are better. There is a space for time tradeoff, and consideration of whether the data will be updated, especially if there is a good chance that it will grow.
There is a difference between early performance optimization and using a best practice type of rule. If you are creating new tables where you will always have a fixed length field, it makes sense to use CHAR, you should be using it in that case. This isn't early optimization, but rather implementing a rule of thumb (or best practice).
i.e. - If you have a 2 letter state field, use CHAR(2). If you have a field with the actual state names, use VARCHAR.
I would choose varchar unless the column stores fixed value like US state code -- which is always 2 chars long and the list of valid US states code doesn't change often :).
In every other case, even like storing hashed password (which is fixed length), I would choose varchar.
Why -- char type column is always fulfilled with spaces, which makes for column my_column defined as char(5) with value 'ABC' inside comparation:
my_column = 'ABC' -- my_column stores 'ABC ' value which is different then 'ABC'
false.
This feature could lead to many irritating bugs during development and makes testing harder.
CHAR takes up less storage space than VARCHAR if all your data values in that field are the same length. Now perhaps in 2009 a 800GB database is the same for all intents and purposes as a 810GB if you converted the VARCHARs to CHARs, but for short strings (1 or 2 characters), CHAR is still a industry "best practice" I would say.
Now if you look at the wide variety of data types most databases provide even for integers alone (bit, tiny, int, bigint), there ARE reasons to choose one over the other. Simply choosing bigint every time is actually being a bit ignorant of the purposes and uses of the field. If a field simply represents a persons age in years, a bigint is overkill. Now it's not necessarily "wrong", but it's not efficient.
But its an interesting argument, and as databases improve over time, it could be argued CHAR vs VARCHAR does get less relevant.
I would NEVER use chars. I’ve had this debate with many people and they always bring up the tired cliché that char is faster. Well I say, how much faster? What are we talking about here, milliseconds, seconds and if so how many? You’re telling me because someone claims its a few milliseconds faster, we should introduce tons of hard to fix bugs into the system?
So here are some issues you will run into:
Every field will be padded, so you end up with code forever that has RTRIMS everywhere. This is also a huge disk space waste for the longer fields.
Now let’s say you have the quintessential example of a char field of just one character but the field is optional. If somebody passes an empty string to that field it becomes one space. So when another application/process queries it, they get one single space, if they don’t use rtrim. We’ve had xml documents, files and other programs, display just one space, in optional fields and break things.
So now you have to ensure that you’re passing nulls and not empty string, to the char field. But that’s NOT the correct use of null. Here is the use of null. Lets say you get a file from a vendor
Name|Gender|City
Bob||Los Angeles
If gender is not specified than you enter Bob, empty string and Los Angeles into the table. Now lets say you get the file and its format changes and gender is no longer included but was in the past.
Name|City
Bob|Seattle
Well now since gender is not included, I would use null. Varchars support this without issues.
Char on the other hand is different. You always have to send null. If you ever send empty string, you will end up with a field that has spaces in it.
I could go on and on with all the bugs I’ve had to fix from chars and in about 20 years of development.
I stand by Jim McKeeth's comment.
Also, indexing and full table scans are faster if your table has only CHAR columns. Basically the optimizer will be able to predict how big each record is if it only has CHAR columns, while it needs to check the size value of every VARCHAR column.
Besides if you update a VARCHAR column to a size larger than its previous content you may force the database to rebuild its indexes (because you forced the database to physically move the record on disk). While with CHAR columns that'll never happen.
But you probably won't care about the performance hit unless your table is huge.
Remember Djikstra's wise words. Early performance optimization is the root of all evil.
Many people have pointed out that if you know the exact length of the value using CHAR has some benefits. But while storing US states as CHAR(2) is great today, when you get the message from sales that 'We have just made our first sale to Australia', you are in a world of pain. I always send to overestimate how long I think fields will need to be rather than making an 'exact' guess to cover for future events. VARCHAR will give me more flexibility in this area.
I think in your case there is probably no reason to not pick Varchar. It gives you flexibility and as has been mentioned by a number of respondants, performance is such now that except in very specific circumstances us meer mortals (as opposed to Google DBA's) will not notice the difference.
An interesting thing worth noting when it comes to DB Types is the sqlite (a popular mini database with pretty impressive performance) puts everything into the database as a string and types on the fly.
I always use VarChar and usually make it much bigger than I might strickly need. Eg. 50 for Firstname, as you say why not just to be safe.
It's the classic space versus performance tradeoff.
In MS SQL 2005, Varchar (or NVarchar for lanuagues requiring two bytes per character ie Chinese) are variable length. If you add to the row after it has been written to the hard disk it will locate the data in a non-contigious location to the original row and lead to fragmentation of your data files. This will affect performance.
So, if space is not an issue then Char are better for performance but if you want to keep the database size down then varchars are better.
Fragmentation. Char reserves space and VarChar does not. Page split can be required to accommodate update to varchar.
There is some small processing overhead in calculating the actual needed size for a column value and allocating the space for a Varchar, so if you are definitely sure how long the value will always be, it is better to use Char and avoid the hit.
when using varchar values SQL Server needs an additional 2 bytes per row to store some info about that column whereas if you use char it doesn't need that
so unless you
Using CHAR (NCHAR) and VARCHAR (NVARCHAR) brings differences in the ways the database server stores the data. The first one introduces trailing blanks; I have encountered problem when using it with LIKE operator in SQL SERVER functions. So I have to make it safe by using VARCHAR (NVARCHAR) all the times.
For example, if we have a table TEST(ID INT, Status CHAR(1)), and you write a function to list all the records with some specific value like the following:
CREATE FUNCTION List(#Status AS CHAR(1) = '')
RETURNS TABLE
AS
RETURN
SELECT * FROM TEST
WHERE Status LIKE '%' + #Status '%'
In this function we expect that when we put the default parameter the function will return all the rows, but in fact it does not. Change the #Status data type to VARCHAR will fix the issue.
In some SQL databases, VARCHAR will be padded out to its maximum size in order to optimize the offsets, This is to speed up full table scans and indexes.
Because of this, you do not have any space savings by using a VARCHAR(200) compared to a CHAR(200)