CASE expression to Replace NULLs with strings in SQL confusion [closed] - sql

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I have 2 columns one is complete and the second one includes missing values as NULL
Like this
table, th, td {
border: 0.4px solid black;
}
table {
width: 50%;
}
th {
height: 30px;
}
<table>
<tr>
<th>column1</th>
<th> column2</th>
</tr>
<tr>
<td>late fee</td>
<td> X</td>
</tr>
<tr>
<td>billing disbute</td>
<td> Y</td>
</tr>
<tr>
<td>transiction issue</td>
<td> NULL</td>
</tr>
<tr>
<td>late fee</td>
<td>NULL</td>
</tr>
<tr>
<td>billing disbute</td>
<td>Y</td>
</tr>
<tr>
<td>transiction issue</td>
<td>X</td>
</tr>
<tr>
<td>billing disbute</td>
<td>NULL</td>
</tr>
<tr>
<td>late fee</td>
<td>X</td>
</tr>
</table>
I checked and each distinct value of column1 has either X or Y in column2
So my approach was to create a table include distinct values of column1 and column2 excluding NULLs
AS in this query
SELECT DISTINCT coulmn1, column2
FROM db
WHERE column2 IS NOT NULL
table, th, td {
border: 0.4px solid black;
}
table {
width: 50%;
}
th {
height: 30px;
}
<table>
<tr>
<th>column1</th>
<th> column2</th>
</tr>
<tr>
<td>billing disbute</td>
<td>Y</td>
</tr>
<tr>
<td>transiction issue</td>
<td>X</td>
</tr>
<tr>
<td>late fee</td>
<td>X</td>
</tr>
</table>
THEN use REPLACE function nested in a CASE expression with 2 conditions
This is the query I used
WITH t2 AS (
SELECT DISTINCT coulmn1, column2
FROM db
WHERE column2 IS NOT NULL )
SELECT db.column1,
CASE WHEN db.column2 IS NULL AND db.column1 = t1.column1
THEN REPLACE ('column2', 'NULL', 't2.column2') END
FROM db, t2
And I was expecting something like this table with no NULL
table, th, td {
border: 0.4px solid black;
}
table {
width: 50%;
}
th {
height: 30px;
}
<table>
<tr>
<th>column1</th>
<th> column2</th>
</tr>
<tr>
<td>late fee</td>
<td> X</td>
</tr>
<tr>
<td>billing disbute</td>
<td> Y</td>
</tr>
<tr>
<td>transiction issue</td>
<td> X</td>
</tr>
<tr>
<td>late fee</td>
<td>X</td>
</tr>
<tr>
<td>billing disbute</td>
<td>Y</td>
</tr>
<tr>
<td>transiction issue</td>
<td>X</td>
</tr>
<tr>
<td>billing disbute</td>
<td>Y</td>
</tr>
<tr>
<td>late fee</td>
<td>X</td>
</tr>
</table>
The issue is this query replaced all values in column2 to NULLs
which the complete opposite of what it supposed to do
and the output was like this instead
table, th, td {
border: 0.4px solid black;
}
table {
width: 50%;
}
th {
height: 30px;
}
<table>
<tr>
<th>column1</th>
<th> column2</th>
</tr>
<tr>
<td>late fee</td>
<td>NULL</td>
</tr>
<tr>
<td>billing disbute</td>
<td>NULL</td>
</tr>
<tr>
<td>transiction issue</td>
<td> NULL</td>
</tr>
<tr>
<td>late fee</td>
<td>NULL</td>
</tr>
<tr>
<td>billing disbute</td>
<td>NULL</td>
</tr>
<tr>
<td>transiction issue</td>
<td>NULL</td>
</tr>
<tr>
<td>billing disbute</td>
<td>NULL</td>
</tr>
<tr>
<td>late fee</td>
<td>NULL</td>
</tr>
</table>