Question

let’s assume we have two tables

table1
    ----------------------
    |ID         | Date   |
    ----------------------
    |1          |20110101|
    |1          |20110102|
    |1          |20110103|
    |2          |20110102|
    |2          |20110103|
    |2          |20110104|
    ----------------------

table2
    ----------------------
    |ID2        |val     |
    ----------------------
    |1          |152     |
    |2          |155     |
    ----------------------

Using this query

SELECT * FROM table1, table2
WHERE table1.ID = table2.ID2
GROUP BY table1.ID 
ORDER BY DATE DESC

Mysql should return this

-------------------------------------------
|ID         |date    |ID2        |val     |
-------------------------------------------
|1          |20110103|1          |152     |
|2          |20110104|2          |155     |
-------------------------------------------

In Oracle I get this error:
ORA-00979: not a GROUP BY expression

EDIT:
The MAX function on the column Date does not work because this column is varchar(200)
The database/tables structure is not mine and I cannot alter it.

Answer 1

You need to do one of two things…

• GROUP BY and use aggregate functions to consolidate multiple records down to one
• Use some lookup to identify the one record you want from the group

In your case, you don’t just want a MAX() from table1, as it may be possible that a higher id has a lower date. In that case, I’d be inclined to use a lookup system…

WITH
  ordered_table1 AS
(
  SELECT
    ROW_NUMBER() OVER (PARTITION BY id ORDER BY date_field DESC) AS sequence_number,
    *
  FROM
    table1
)
SELECT
  *
FROM
  ordered_table1
INNER JOIN
  table2
    ON table2.id = ordered_table1.id
WHERE
  ordered_table1.sequence_id = 1

NOTE: This assumes your date is formatted such that alphanumeric ordering WILL yield the correct date order. If that is NOT the case (and d-m-yyyy will not order correctly), you need to replace date_field with TO_DATE(date_field) to ensure the correct order.

NOTE: Use of TO_DATE(date_field) will also probably fix your MAX() problems.

NOTE: If you want to store dates as strings, but them to be order friendly, use yyyy-mm-dd