ORA-00998: must name this expression with a column alias - sql

Question

I get that I should add alias with all the columns and I'm doing so but I'm still getting error.
CREATE TABLE MENTIONS AS SELECT
UM.USER_ID AS U_ID,
UM.SCREEN_NAME AS USER_SCREEN_NAME,
UM.MENTION_ID AS M_USER_ID,
(
SELECT
UI.USER_SCREEN_NAME AS MENTIONED_USER
FROM
USER_INFO UI
WHERE
UI.USER_ID = UM.MENTION_ID
AND ROWNUM = 1
)
FROM
USER_MENTION UM
USER_MENTION table
USER_ID SCREEN_NAME MENTION_ID
135846337 irisschrijft 774759032636727300
50117969 Chjulian 13769472
14411827 thenriques45 13769472
26681613 ahenotri 252074645
26681613 ahenotri 9796472
158378782 SpringerOpen 9796472
144241914 Kumarappan 252074645
User_INFO table:
USER_ID USER_SCREEN_NAME
22553325 jasonesummers
23435691 QRJAM false
67421923 inTELEgentMSP
97393397 knauer0x
85303739 MarriageTheorem
3842711 seki
3036414608 Bayes_Rule
838677852 BOLIGATOR
I'm still getting the above mentioned error, what am I doing wrong?

Answer 1

Lookup the Oracle Error Message Manual of the current Oracle version. Here the error is mentioned but without additional information.
In such a case look up the
Oracle Error Message Manual of version 9i
For reasons I don't know a lot of error messages have a description in the 9i manual but not in the manuals of higher versions. 9i is a rather old version so the description may be out of date. But it may contain valuable hints.
ORA-00998 must name this expression with a column alias
Cause: An expression or function was used in a CREATE VIEW statement, but no corresponding column name was specified. When expressions or functions are used in a view, all column names for the view must be explicitly specified in the CREATE VIEW statement.
Action: Enter a column name for each column in the view in parentheses after the view name.
We don't have a view but a a table that was created by a select. And actually the last expression of the select list is an expression without an alias. So try your statement using an alias for the last expression. So try
CREATE TABLE MENTIONS AS SELECT
UM.USER_ID AS U_ID,
UM.SCREEN_NAME AS USER_SCREEN_NAME,
UM.MENTION_ID AS M_USER_ID,
(
SELECT
UI.USER_SCREEN_NAME
FROM
USER_INFO UI
WHERE
UI.USER_ID = UM.MENTION_ID
AND ROWNUM = 1
) AS MENTIONED_USER
FROM
USER_MENTION UM
The column alias in the inner select list is useless and can be removed.

Answer 2

The problem with your query is that each column in the create table needs to have a name. You think you are assigning a name in the sub-select. However, you are not.
The subquery is just returning a value -- not a value with a name. So, the AS MENTIONED_USER in your version does nothing. This is a bit tricky, I guess. One way to think of the scalar subquery is that it is just another expression or function call. Things that happen inside it don't affect the outer query -- except for the value being returned.
The correct syntax is to put the column alias outside the subselect, not inside it:
CREATE TABLE MENTIONS AS
SELECT UM.USER_ID AS U_ID, UM.SCREEN_NAME AS USER_SCREEN_NAME, UM.MENTION_ID AS M_USER_ID,
(SELECT UI.USER_SCREEN_NAME
FROM USER_INFO UI
WHERE UI.USER_ID = UM.MENTION_ID AND ROWNUM = 1
) AS MENTIONED_USER
FROM USER_MENTION UM;