查询字段区分度

查询字段区分度，NUM_DISTINCT值越大，唯一度越高，区分度越大

SELECT column_name,
          DECODE (t.data_type,
                  'NUMBER', t.data_type
                   || '('
                   || DECODE (t.data_precision,
                              NULL, t.data_length || ')',
                              t.data_precision || ',' || t.data_scale || ')'
                             ),
                  'DATE', t.data_type,
                  'LONG', t.data_type,
                  'LONG RAW', t.data_type,
                  'ROWID', t.data_type,
                  'MLSLABEL', t.data_type,
                  t.data_type || '(' || t.data_length || ')'
                 )
       || ' '
       || DECODE (t.nullable, 'N', 'NOT NULL', 'n', 'NOT NULL', NULL) col,
       num_distinct, density
  FROM dba_tab_columns t
 WHERE table_name = UPPER ('&Table_name')
   AND owner = UPPER (NVL ('&Owner', USER));