try to concatenate 2 strings, result ends in a lot of spaces - sql

Question

select CONCAT(convert(char, 123), 'sda');
Or
select convert(char, 123) + 'sda'
Or
select ltrim(convert(char, 123) + 'sda')
Output is:
How can I get the output without those spaces?

Answer 1

The problem here is 2 fold. Firstly that you are converting to a char, which is a fixed width datatype, and secondly that you aren't defining the length of your char, therefore the default length is used. For CAST and CONVERT that's a char(30).
So, what you have to start is convert(char, 123). This converts the int 123 to the fixed width string '123 '. Then you concatenate the varchar(3) value 'sda' to that, resulting in '123 sda'. This is working exactly as written, but clearly not as you intend.
The obvious fix would be to use a varchar and define a length, such as CONCAT(CONVERT(varchar(5),123),'sda') which would return '123sda', however, all of the CONCAT function's parameters are a string type:
string_value
A string value to concatenate to the other values. The CONCAT function requires at least two string_value arguments, and no more than 254 string_value arguments.
This means you can simply just pass the value 123 and it'll be implicitly cast to a string type: CONCAT(123,'sda').
To reiterate my comment's link too: Bad Habits to Kick : Declaring VARCHAR without (length)

Answer 2

You are using char while you probably want [n]varchar(...): the former pads the string with white spaces, while the latter does not:
concat(convert(varchar(10), 123), 'sda');
But simpler yet: concat() forces the conversion of its arguments to the correct datatype by default, so this should do it:
concat(123, 'sda')

Answer 3

First, in SQL Server, never us char or related string definitions without a length. SQL Server requires a length and the default depends on the context. If you depend on the default length your code has a bug just waiting to happen.
Second, char is almost never what you want. It is a fixed length string, with shorter strings padded with spaces.
If you want an explicit conversion use varchar, variable length strings:
select convert(varchar(255), 123) + 'sda'
Or dispense with the explicit conversion and use concat():
select concat(123, 'sda')

Answer 4

As the others have already pointed out the root cause of the issue, if you cannot edit the datatype, you can always use SELECT CONCAT(TRIM(CONVERT(char,123)),'sda'). Although it's highly recommended to either use varchar(n) or give exclusive length of char as it is kind of pointless to create fixed length string and then reduce the length by using TRIM. varchar(30) would perfectly fit in here as the length can still NOT exceed the 30 symbols, but would not use all the length if the string is shorter.

Answer 5

Lets refer to Microsoft docs:
When n isn't specified in a data definition or variable declaration statement, the default length is 1. If n isn't specified when using the CAST and CONVERT functions, the default length is 30.
Reference: https://learn.microsoft.com/en-us/sql/t-sql/data-types/char-and-varchar-transact-sql?view=sql-server-ver15#remarks
So, You have Convert(char, 123), and you did not specify the n for char, so your code is equal to Convert(char(30), 123).
Now it is clear why you have many space characters. To resolve the problem simply use variant length character datatypes such as varchar instead, however I recommend you to always use character datatypes with length. (Same as what #GordonLinoff posted: https://stackoverflow.com/a/63467483/1666800)
select convert(varchar, 123) + 'sda'