问题
My db table is called transactions and is like this:
Name | Date (DateTime) | Type | Stock | Volume | Price | Total
Tom 2014-05-24 12:00:00 Sell Barclays 100 2.2 220.0
Bob 2014-04-13 15:00:00 Buy Coca-Cola 10 12.0 120.0
varchar DateTime varchar varchar int float float
My initial problem was to remove from the table ALL the transactions that belong to a user whose first transaction is later than a certain threshold. My query was:
DELETE FROM transactions WHERE name NOT IN (SELECT name FROM transactions2 WHERE date < CAST('2014-01-01 12:00:00.000' as DateTime));
Query OK, 35850 rows affected (3 hours 5 min 28.88 sec)
I think this is a poor solution, I had to duplicate the table to avoid deleting from the same table from where I am reading, and the execution took quite a long time (3 hours for a table containing ~170k rows)
Now I am trying to delete ALL the transactions that belong to a user whose latest transaction happened before a certain threshold date.
DELETE FROM transactions WHERE name IN (SELECT name FROM transactions HAVING max(date) < CAST('2015-01-01 12:00:00.000' as DateTime) );
Sadly, the subquery finds only one result:
SELECT name FROM transactions HAVING max(date) < CAST('2015-01-01 12:00:00.000' as DateTime)';
+------------+
| name |
+------------+
| david |
+------------+
I guess I am getting only one result because of the max() function. I am not an expert of SQL but I understand quite well what I need in terms of sets and logic. I would be really happy to have suggestions on how to rewrite my query.
EDIT: Here is a sqlfiddle with the schema and some data: http://sqlfiddle.com/#!2/389ede/2
I need to remove ALL the entries for alex, because his last transactions happened before a certain threshold (let's say 1 Jan 2013). Don't need to delete tom's transactions because he has his latest later than 1 Jan 2013.
回答1:
Your first query can be formulated as: `delete users from transactions where it does not exist a transaction for that user before ?. This is easy to transform to sql:
delete from transactions t1
where not exists (
select 1 from transactions t2
where t1.name = t2.name
and t2.date < ?
)
mysql still does not support (AFAIK) deleting from a table that is referenced in a select, so we need to rewrite it as:
delete t1.*
from transactions t1
left join transactions t2
on t1.name = t2.name
and t2.date < ?
where t2.name is null
date is a reserved word so you will have to quote that.
Your second query can be solved the same way, delete from transaction where it does not exists a transaction after a certain date. I'll leave it as an exercise.
回答2:
Alvin here is a simplified scenario from your fiddle with dates:
CREATE TABLE transactions
( id int(11) NOT NULL AUTO_INCREMENT
, name varchar(30) NOT NULL
, value datetime NOT NULL
, PRIMARY KEY (id) ) ENGINE=InnoDB;
INSERT INTO transactions (name, value) VALUES ('alex', '2011-01-01 12:00:00')
, ('alex', '2012-06-01 12:00:00');
Let's investigate what happens in:
SELECT t1.name as t1_name, t1.value as t1_value
, t2.name as t2_name, t2.values as t2_value
FROM transactions t1
LEFT JOIN transactions t2
ON t1.name = t2.name
T1_NAME T1_VALUE T2_NAME T2_VALUE
alex January, 01 2011 12:00:00+0000 alex January, 01 2011 12:00:00+0000
alex January, 01 2011 12:00:00+0000 alex June, 01 2012 12:00:00+0000
alex June, 01 2012 12:00:00+0000 alex January, 01 2011 12:00:00+0000
alex June, 01 2012 12:00:00+0000 alex June, 01 2012 12:00:00+0000
I.e. 4 rows. If we now add the join predicate:
SELECT t1.name as t1_name, t1.value as t1_value
, t2.name as t2_name, t2.values as t2_value
FROM transactions t1
LEFT JOIN transactions t2
ON t1.name = t2.name
AND t2.value > CAST('2011-06-01 12:00.000' as DateTime)
This leaves us with two rows. If we change the time to '2012-06-01 12:00.000' we still have two rows due to the left join, but the t2 columns will be null.
If we now add the WHERE clause:
SELECT t1.name as t1_name, t1.value as t1_value
, t2.name as t2_name, t2.values as t2_value
FROM transactions t1
LEFT JOIN transactions t2
ON t1.name = t2.name
AND t2.value > CAST('2012-06-01 12:00.000' as DateTime)
WHERE t2.name is null;
we still have two rows. With CAST('2011-06-01 12:00.000' as DateTime) there are no rows.
Remember that the construction is equivalent with:
SELECT t1.name as t1_name, t1.value as t1_value
FROM transactions t1
WHERE NOT EXISTS (
SELECT 1 FROM transactions t2
WHERE t1.name = t2.name
AND t2.value > CAST('2012-06-01 12:00.000' as DateTime)
);
So, if it does not exist a row for the name where value > '2012-06-01 12:00.000' we have a match. Does that clarify?
回答3:
@Lennart, Alvin, consider the following...
DROP TABLE IF EXISTS my_table;
CREATE TABLE my_table (id INT NOT NULL AUTO_INCREMENT PRIMARY KEY,val INT NOT NULL);
INSERT INTO my_table (val) VALUES (1),(1),(2),(1),(3),(2),(3),(1),(4);
SELECT * FROM my_table;
+----+-----+
| id | val |
+----+-----+
| 1 | 1 |
| 2 | 1 |
| 3 | 2 |
| 4 | 1 |
| 5 | 3 |
| 6 | 2 |
| 7 | 3 |
| 8 | 1 |
| 9 | 4 |
+----+-----+
Let's delete the most recent result for each val, i.e. the result of...
SELECT x.*
FROM my_table x
JOIN
( SELECT val, max(id) max_id FROM my_table GROUP BY val ) y
ON y.val = x.val
AND y.max_id = x.id;
+----+-----+
| id | val |
+----+-----+
| 8 | 1 |
| 6 | 2 |
| 7 | 3 |
| 9 | 4 |
+----+-----+
So...
DELETE x
FROM my_table x
JOIN ( SELECT val, max(id) max_id FROM my_table GROUP BY val ) y
ON y.val = x.val
AND y.max_id = x.id;
SELECT * FROM my_table;
+----+-----+
| id | val |
+----+-----+
| 1 | 1 |
| 2 | 1 |
| 3 | 2 |
| 4 | 1 |
| 5 | 3 |
+----+-----+
来源：https://stackoverflow.com/questions/23935467/sql-query-based-on-subquery-retrieve-transactions-with-data-threshold