Find all columns having at least a NULL value from all tables in the schema

Spending a good amount of time answering the questions on multiple Oracle forums, like OraFAQ, Stack Overflow, OTN, dBforums etc. I could now figure out the most common questions by amateurs in Oracle SQL and PL/SQL. My recent blog posts are focused on such questions. And one among them is “How to find all columns having at least a NULL value from all tables in the schema“.

Now, in SQL it would be a huge task to manually code all the column names in the WHERE clause to check IS NULL condition. However, the manual effort could be minimized to a great extent using an UDF(user defined function) and query on dynamic view ALL_TAB_COLUMNS. Let me demonstrate using the SCOTT schema :

• Create a simple function FIND_NULL_COL, it would return 1 if found any NULL value in any column.

SQL> CREATE
  2  OR REPLACE FUNCTION FIND_NULL_COL(TABLE_NAME VARCHAR2, COLUMN_NAME VARCHAR2)
  3  RETURN NUMBER
  4  IS cnt NUMBER;
  5  BEGIN
  6    cnt :=1;
  7    EXECUTE immediate 'select count(1) from '||table_name||' where '||column_name||' is not null' INTO cnt;
  8    RETURN
  9    CASE
 10    WHEN CNT=0 THEN
 11      1
 12    ELSE
 13      0
 14    END;
 15  END;
 16  /

Function created.

• Now, use the UDF in your SQL.

SQL> set pagesize 1000
SQL> column owner format A10;
SQL> column column_name format A20;
SQL> COLUMN TABLE_NAME FORMAT A20;
SQL> column n format A1;
SQL> SELECT c.OWNER,
  2    c.TABLE_NAME,
  3    c.COLUMN_NAME,
  4    C.NULLABLE,
  5    FIND_NULL_COL(c.TABLE_NAME,c.COLUMN_NAME) null_status
  6  FROM all_tab_columns c
  7  WHERE c.OWNER=USER
  8  AND c.TABLE_NAME LIKE '%'
  9  ORDER BY C.OWNER,
 10    C.TABLE_NAME,
 11    C.COLUMN_ID
 12  /

OWNER      TABLE_NAME           COLUMN_NAME          N NULL_STATUS
---------- -------------------- -------------------- - -----------
SCOTT      BONUS                ENAME                Y           1
SCOTT      BONUS                JOB                  Y           1
SCOTT      BONUS                SAL                  Y           1
SCOTT      BONUS                COMM                 Y           1
SCOTT      DEPT                 DEPTNO               N           0
SCOTT      DEPT                 DNAME                Y           0
SCOTT      DEPT                 LOC                  Y           0
SCOTT      EMP                  EMPNO                N           0
SCOTT      EMP                  ENAME                Y           0
SCOTT      EMP                  JOB                  Y           0
SCOTT      EMP                  MGR                  Y           0
SCOTT      EMP                  HIREDATE             Y           0
SCOTT      EMP                  SAL                  Y           0
SCOTT      EMP                  COMM                 Y           0
SCOTT      EMP                  DEPTNO               Y           0
SCOTT      SALGRADE             GRADE                Y           0
SCOTT      SALGRADE             LOSAL                Y           0
SCOTT      SALGRADE             HISAL                Y           0

18 rows selected.

SQL>

So, NULL_STATUS “1” is the list of table names and respective column names which has at least a NULL value.

You could also add more filters to the where clause :

SQL> CREATE OR REPLACE
  2    FUNCTION FIND_NULL_COL(
  3        TABLE_NAME  VARCHAR2,
  4        COLUMN_NAME VARCHAR2)
  5      RETURN NUMBER
  6    IS
  7      cnt NUMBER;
  8    BEGIN
  9      CNT :=1;
 10      EXECUTE IMMEDIATE 'select count(1) from '
 11                        ||TABLE_NAME||' where ' ||COLUMN_NAME||' is not null
 12                        and deptno = 20' INTO cnt;
 13      RETURN
 14      CASE
 15      WHEN CNT=0 THEN
 16        1
 17      ELSE
 18        0
 19      END;
 20    END;
 21    /

Function created.

SQL>   SET pagesize 1000
SQL>   column owner format A10;
SQL>   column column_name format A20;
SQL>   COLUMN TABLE_NAME FORMAT A20;
SQL>   column n format A1;
SQL>   SELECT c.OWNER,
  2      c.TABLE_NAME,
  3      c.COLUMN_NAME,
  4      C.NULLABLE,
  5      FIND_NULL_COL(c.TABLE_NAME,c.COLUMN_NAME) null_status
  6    FROM all_tab_columns c
  7    WHERE C.OWNER    =USER
  8    AND c.TABLE_NAME = 'EMP'
  9    ORDER BY C.OWNER,
 10      C.TABLE_NAME,
 11      C.COLUMN_ID
 12  /

OWNER      TABLE_NAME           COLUMN_NAME          N NULL_STATUS
---------- -------------------- -------------------- - -----------
SCOTT      EMP                  EMPNO                N           0
SCOTT      EMP                  ENAME                Y           0
SCOTT      EMP                  JOB                  Y           0
SCOTT      EMP                  MGR                  Y           0
SCOTT      EMP                  HIREDATE             Y           0
SCOTT      EMP                  SAL                  Y           0
SCOTT      EMP                  COMM                 Y           1
SCOTT      EMP                  DEPTNO               Y           0

8 rows selected.

SQL>

If you do not want to create the function, then there is another way to get the list of columns having NULL value. Using the *_TAB_COLUMNS view. However, you MUST gather statistics before querying the view to get accurate results.

SQL> EXEC DBMS_STATS.gather_schema_stats('SCOTT');

PL/SQL procedure successfully completed.

SQL>
SQL> SELECT t.table_name, t.column_name
 2 FROM user_tab_columns t
 3 WHERE t.nullable = 'Y'
 4 AND t.num_distinct = 0;

TABLE_NAME COLUMN_NAME
---------- ------------
BONUS ENAME
BONUS JOB
BONUS SAL
BONUS COMM
CLASS NAME
CLASS TIME
CLASS ROOM
CLASS FID

8 rows selected.

SQL>

Hope it helps!