SQL DB2 Replace value in a column

Question

I have the following column, B represents boolean and the rest are empty values. I have to change all the values in this column to the word COLUMN A.

  COLUMN
  -----


   B

I have tried different things, for example

 SELECT COLUMN 
 FROM TABLE
 WHERE COALESCE(NULLIF(COLUMN,''), 'COLUMN A');

And I receive the error: "Invalid character found in a character string argument of the function "BOOLEAN"." I'm kind of stuck to this question and I'm getting confused with this boolean value. I will be really happy if someone can help me, thanks!

Answer 1

The WHERE clause is unfinished. Compare the COALESCEd value to something:

SELECT COLUMN 
 FROM TABLE
 WHERE COALESCE(NULLIF(COLUMN,''), 'COLUMN A') = 'COLUMN A';

Or better:

SELECT COLUMN 
 FROM TABLE
 WHERE COLUMN IS NULL OR COLUMN = ''

Doesn't require any thinking/calculating to work out your selection logic. More maintainable, nicer for peer developers

*The above is generic advice for usual cases NOT involving boolean datatypes (which typically require some different treatment)

---

Now, you say you have to change the value to something. That requires an UPDATE statement. If this column is a boolean then it won't have a value of empty string. The blanks will be nulls:

UPDATE TABLE SET COLUMN = (some boolean) WHERE COLUMN IS NULL

If you don't want to permanently change the table data to something, but instead want to select it out as some value where a blank occurs, but keep the blanks stored in the table:

SELECT COALESCE(column, (some boolean)) FROM TABLE

Might be worth noting that not all versions of DB2 can return a boolean in a result set - this is quite typical of database vendors. Convert the boolean to something else representable using a case when, if your DB2 version is thus restricted

SELECT CASE WHEN column = TRUE THEN 'true' ELSE 'false' END FROM TABLE

Answer 2

The easiest thing is to use CASE expression. I am not familiar in db2, so you may want to research it further, but in other DBMSs it works like this:

SELECT CASE WHEN COLUMN = '' THEN 'COLUMN A' -- if COLUMN = '', replace it with 'COLUMN A' ELSE COLUMN -- otherwise, keep COLUMN as is. END as 'COLUMN' -- name the column in the result 'COLUMN' FROM TABLE

This is an article that explains how it works in db2: https://www.ibm.com/support/knowledgecenter/en/SSEPEK_11.0.0/sqlref/src/tpc/db2z_caseexpression.html