Understanding the Problem and Initial Query Attempt

When joining multiple tables using LEFT JOINs, it’s common to encounter duplicate rows. In this case, we’re dealing with three tables: a, b, and x. Table a has a starting position, field length, FUID, and format columns, while tables b and x have FNAME, FUID, RID, IND, FLAG, and CHAR columns.

The original query attempts to solve the problem by joining table a with the result of another LEFT JOIN between table b and an alias for table x. The join conditions are based on matching FUID values between tables a and b, as well as RID values between table a and table x.

However, this approach leads to duplicate rows due to cross multiplication during the joins. Additionally, the query doesn’t address the specific requirement of assigning ‘S’ or ‘F’ status based on matching or mismatching values in the CHAR column.

The Problem with Left Joining Three or More Tables

When joining three or more tables using LEFT JOINs, each join operation can result in multiple rows for a single row from the previous table. This is especially true when dealing with cross joins or left joins with multiple conditions.

In our case, we’re trying to join table x with table a, which will produce 16 possible combinations due to the presence of RID and IND columns in both tables. The resulting rows may not be unique, leading to duplicate values in the output.

Using Row_Number() Function

One solution is to use the ROW_NUMBER() function to eliminate duplicate rows based on specific conditions. In this approach, we can assign a unique number to each row within a partition of a result set.

For example:

SELECT *
FROM (
  SELECT a.STEP_ID,
         b.FNAME,
         a.FORMAT,
         x.RID,
         x.IND,
         x.FLAG,
         x.CHAR,
         (a.START_POSITION - lag(a.START_POSITION + a.FIELD_LENGTH,1,1) OVER (ORDER BY a.START_POSITION)) AS BLANK
  FROM s1.a a
  LEFT JOIN si.b b ON b.FUID = a.FUID
  LEFT JOIN x ON x.RID = a.STEP_ID
  WHERE a.STEPID = 1
) t
ROW_NUMBER() OVER (PARTITION BY FNAME ORDER BY FUID) AS rn
WHERE rn = 1;

By using ROW_NUMBER(), we assign a unique row number to each row within the partition based on the FNAME value. We can then select only rows with rn = 1, effectively eliminating duplicate values.

Joining Tables Using RowNumber()

Another approach is to join tables using ROW_NUMBER(). In this method, we use ROW_NUMBER() to identify unique combinations of values from multiple columns.

For example:

SELECT t.STEP_ID, t.FNAME, t.FORMAT,
       x.RID, x.IND, x.FLAG, x.CHAR,
       t.BLANK,
       CASE WHEN t.blank = x.CHARS THEN 'S' ELSE 'F' END STATUS
FROM (
  SELECT a.STEP_ID, b.FNAME, a.FORMAT, 
         (a.START_POSITION - lag(a.START_POSITION + a.FIELD_LENGTH,1,1) OVER (ORDER BY a.START_POSITION)) AS BLANK,
         rownum as global_id FROM taba a
  LEFT JOIN tabb b ON a.FUID = b.FUID
) t
JOIN (
  SELECT RID, IND, FLAG, CHARS,
         rownumber() OVER (PARTITION BY CHARS ORDER BY CHARS) AS rid2
  FROM tabx
) x ON t.STEP_ID = x.RID AND t.global_id = x.rid2;

In this approach, we use ROW_NUMBER() to assign a unique number to each combination of values from multiple columns in table x. We then join the result with the original query using the RID value and the row number.

Assigning ‘S’ or ‘F’ Status

To address the requirement of assigning ‘S’ or ‘F’ status based on matching or mismatching values in the CHAR column, we can use a CASE statement within our query.

For example:

SELECT t.STEP_ID, t.FNAME, t.FORMAT,
       x.RID, x.IND, x.FLAG, x.CHAR,
       t.BLANK,
       CASE WHEN t.blank = x.CHARS THEN 'S' ELSE 'F' END STATUS
FROM (
  SELECT a.STEP_ID, b.FNAME, a.FORMAT, 
         (a.START_POSITION - lag(a.START_POSITION + a.FIELD_LENGTH,1,1) OVER (ORDER BY a.START_POSITION)) AS BLANK,
         rownum as global_id FROM taba a
  LEFT JOIN tabb b ON a.FUID = b.FUID
) t
JOIN (
  SELECT RID, IND, FLAG, CHARS,
         rownumber() OVER (PARTITION BY CHARS ORDER BY CHARS) AS rid2
  FROM tabx
) x ON t.STEP_ID = x.RID AND t.global_id = x.rid2;

This approach ensures that the ‘S’ or ‘F’ status is assigned correctly based on matching or mismatching values in the CHAR column.

Conclusion

Joining multiple tables using LEFT JOINs can lead to duplicate rows due to cross multiplication. To address this issue, we can use the ROW_NUMBER() function to eliminate duplicate rows based on specific conditions. Additionally, we can join tables using ROW_NUMBER() and assign ‘S’ or ‘F’ status based on matching or mismatching values in the CHAR column.

By understanding the problem and using appropriate solutions, we can effectively resolve duplicate rows and ensure accurate results when joining multiple tables using LEFT JOINs.

Last modified on 2024-06-27