ORDER BY
Order select output by one or more columns.
CREATE TABLE t (c0 INT, c1 INT);
INSERT INTO t VALUES (0, 1), (0, 0), (1, 1), (2, 0);
# Partially unspecified:
SELECT * FROM t ORDER BY c0;
SELECT * FROM t ORDER BY c0*c1;
# Fully specified:
SELECT * FROM t ORDER BY c0,c1;
SELECT c0 + c1 AS sum FROM t ORDER BY sum;
DROP TABLE t;
Possible output (up to reordering unspecified orders):
c0 c1
0 1
0 0
1 1
2 0
c0 c1
0 1
0 0
2 0
1 1
c0 c1
0 0
0 1
1 1
2 0
sum
0
1
2
2
If not used, the orders are unspecified. Almost all SELECT statements done in practice will have specified order.
This query can be optimized by B-Tree indexes on MySQL and PostgreSQL.