SQL using If Not Null on a Concatenation

Question

If I have the table

SELECT (Firstname || \'-\' || Middlename || \'-\' || Surname)  AS example_column
FROM example_table

This will

Answer 1

• The NULLIF expression reduces blank Middlename to NULL
• Concatenating '-' with a NULL will always return NULL
• TheVALUE expression replaces NULLs with an empty string

_

SELECT Firstname || VALUE( '-' || NULLIF('Middlename',''),'') || '-' || Surname'  
       AS example_column
FROM example_table

Answer 2

If you use Postgres, concat_ws() is what you are looking for:

SELECT concat_ws('-', Firstname, Middlename, Surname)  AS example_column
FROM example_table

SQLFiddle: http://sqlfiddle.com/#!15/9eecb7db59d16c80417c72d1e1f4fbf1/8812

To treat empty strings or strings that only contain spaces like NULL use nullif():

 SELECT concat_ws('-', Firstname, nullif(trim(Middlename), ''), Surname)  AS example_column
 FROM example_table

Answer 3

One solution could be using case statement

select case Middlename is not null then (Firstname || '-' || Middlename || '-' || Surname) 
    else (Firstname || '-' || Surname) end AS example_column
from ....

Answer 4

You could use REPLACE (if Oracle)

SELECT
   REPLACE(Firstname || '-' || Middlename || '-' || Surname,'--','-') 
   AS example_column
FROM example_table;

Warning: I've assumed there is no valid name with - as first or last character.

---

For downvoter

OP clearly said that:

SELECT (Firstname || '-' || Middlename || '-' || Surname)  AS example_column
FROM example_table

This will display Firstname-Middlename-Surname e.g.

John--Smith

So:

1. Middlename is blank: '' this solution works in SQLite/PostgreSQL/Oracle
2. Middlename is NULL and OP probably uses Oracle

Although Oracle treats zero-length character strings as nulls, concatenating a zero-length character string with another operand always results in the other operand, so null can result only from the concatenation of two null strings.

|| concatentaion operator:

 -- PostgreSQL/SQLite
 SELECT 'sth' || NULL
 -- NULL


 -- Oracle
 SELECT 'sth' || NULL
 -- sth

Answer 5

This approach works:

select first_name || coalesce('-' || middle_name, '') || '-' || last_name 
from t;

Output:

|        ?column? |
|-----------------|
|      john-smith |
| jane-anne-smith |

UPDATE

Live code: http://sqlfiddle.com/#!15/d5a1f/1

Just as my inkling, someone will give a scenario that is not in the question. So to make it work with empty middle name. Just add a nullif for empty string:

select first_name || coalesce('-' || nullif(middle_name,'') , '') || '-' || last_name 
from t;

Output:

|        ?column? |
|-----------------|
|      john-smith |
|      obi-kinobi |
| jane-anne-smith |

Answer 6

You can use CASE statement

select Firstname 
      || case when Middlename <> '' Then '-'||Middlename  else '' END 
      || case when Surname<> '' Then '-'||Surname else '' END

As per your sample data I have check for empty string. To check NULL use Middlename IS NOT NULL instead of Middlename <> ''