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.

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