Question

Say I have the following 2 tables,

CREATE TABLE t1(
  name VARCHAR(25) NOT NULL,
  time INT,
  a INT
);

CREATE TABLE t2(
  name VARCHAR(25) NOT NULL,
  time INT,
  b INT
);

and Im looking to pull all the values (a) out of t1 with a given time, all the values with the previous time (say just time-1 for convenience) then for each name subtract the newer one from the older one and insert those values into t2 with the same time. The slow way of doing this would involve doing something like

SELECT name, a FROM t1 WHERE time = x;
SELECT name, a FROM t1 WHERE time = x-1;
(subtract the as for each name)
INSERT INTO t2 VALUES ....;

From my (limited) understanding of subqueries, there should hopefully be a way to do this all in 1 query. Any ideas? Thanks in advance 🙂

Answer 1

It looks like you can use the INSERT ... SELECT syntax:

INSERT INTO t2 (name, time, b)
SELECT  ta.name, ta.time time, (ta.a - tb.a) b
FROM    t1 ta 
JOIN    t1 tb ON (tb.time = ta.time - 1 AND tb.name = ta.name);

Test case:

INSERT INTO t1 VALUES ('t1', 1, 100);
INSERT INTO t1 VALUES ('t1', 2, 200);
INSERT INTO t1 VALUES ('t1', 3, 500);
INSERT INTO t1 VALUES ('t1', 4, 600);
INSERT INTO t1 VALUES ('t1', 5, 800);
INSERT INTO t1 VALUES ('t1', 6, 900);

Result:

SELECT * FROM t2;
+------+------+------+
| name | time | b    |
+------+------+------+
| t1   |    2 |  100 |
| t1   |    3 |  300 |
| t1   |    4 |  100 |
| t1   |    5 |  200 |
| t1   |    6 |  100 |
+------+------+------+
5 rows in set (0.00 sec)