SQL "group by" like - grouping algorithm - sql

Question

I have a table with more than 2 columns (let's say A, B and C). One column holds some numbers (C) and I want to do a "group by" like grouping, summing the numbers in C, but I don't know the algorithm for doing so.
I tried sorting the table by each column (from last to first, aside from the numbers column (C), so in this case: sort(B) and then sort(A)) and then, wherever nth row holds same values in A and B as in n-1th row, I add the number from nth row to n-1th row (in the C column), and then delete the nth row. Else, if A or B value in row n differs from A or B value in n-1th row, I'll just move to the next row. Then I repeat the algorithm till the last row in table. But somehow this isn't working all the time, especially when there're a lot more columns (some rows remain ungrouped, maybe because of the sorting method).
I want to know whether this is a good grouping algorithm and I need to look for the problem into the sorting method, or I need to use another (sorting and/or grouping) algorithm and which one. Thank you.
LE: Apparently the algorithm that I used works well after a thorough check of the code and fixing some minor mistakes that junior programmers like me often make :)

Answer 1

I think a good way to do this would be to wrap your row into a class, implement the equals method, and then use a Map to add the values up:
public class MyRow {
private Long columnA;
private String columnB;
private int columnC;
#Override
public boolean equals(final Object other) {
if (!other instanceof MyRow) {
return false;
}
final MyRow otherRow = (MyRow) other;
return this.columnA.equals(otherRow.getColumnA()) && this.columnB.equals(otherRow.getColumnB);
}
}
Then you can iterate over all the rows, and create a Map for holding the sums of C.
final Map<MyRow, Integer> computedCSums = new HashMap<MyRow, Integer>();
for (final MyRow myRow : myRows) {
if (computedCSums.get(myRow) == null) {
computedCSums.put(myRow, myRow.getColumnC());
} else {
computedCSums.put(myRow, computedSums.get(myRow) + myRow.getColumnC());
}
}
Then, to get the sum of grouped Cs of any row, you just do:
computedCSum.get(mySelectedRow);

Answer 2

I think there is three things should be considered about group by
less or equal is abstract
comparing two rows A, B according it columns (C1..Cn) are like this : compare each column from C1 to Cn , if we can get which is less, then return ,or if the two values are equal, then we go to compare next, repeat this until return.
which algorithm we choose
1)build a binary search tree or a hash table to store tuples , when we get a tuple, search the equal tuple , if we have , then merge the tuple which have the same group value, else put it to our search structure
2) read some tuples, then sort , walk the buffer and merge the same group
I prefer 1 rather than 2.
memory size
if out input is huge, we must consider memory limit.
we can use merge algorithm to deal this.
if memory exceed our limit , then write the tuples in memory to the tape order by their group columns
when we finish reading the input, then merge the result set in tape.