Update Table Contents With Value From The Same Table - mysql

Question

I have a simple table with 10000 rows
product_id | price | price_inc_vat | vat_rate
I want to be able to update the value of price_inc_vat based on the value of price
I have tried this
UPDATE 1_products AS s3,
1_products AS t3
SET t3.price_inc = ROUND(s3.price*1.3,2)
WHERE s3.vat_rate = 20 AND t3.vat_rate=20;
It doesn't error but seems to be taking a very long time as I had to kill the query after 1/2 hour.

Answer 1

Why the join?
UPDATE products SET price_inc_vat = ROUND(price*1.3,2) WHERE vat_rate=20;
UPDATED
I'm not quite sure what you are trying to achieve by updating a product's "Inc. price" with another products price, but the reason that your query is taking so long is because of the way that you are joining.
mysql> create table t1 (product_id integer unsigned primary key, price decimal(10,2), price_inc_vat decimal(10,2), vat_rate integer unsigned);
mysql> insert into t1 values (1, 1.00, 1.20, 20), (2, 2.00, 2.40, 20), (3, 1.00, 1.30, 30);
mysql> select * from (t1 as s3, t1 as t3) where s3.vat_rate=20 and t3.vat_rate=20;
1 1.00 1.20 20 1 1.00 1.20 20
2 2.00 2.40 20 1 1.00 1.20 20
1 1.00 1.20 20 2 2.00 2.40 20
2 2.00 2.40 20 2 2.00 2.40 20
mysql> select s3.*, t3.*, round(s3.price * 1.3, 2) from (t1 as s3, t1 as t3) where s3.vat_rate=20 and t3.vat_rate=20;
1 1.00 1.20 20 1 1.00 1.20 20 1.30
2 2.00 2.40 20 1 1.00 1.20 20 2.60
1 1.00 1.20 20 2 2.00 2.40 20 1.30
2 2.00 2.40 20 2 2.00 2.40 20 2.60
mysql> update (t1 as s3, t1 as t3) set t3.price_inc_vat=round(s3.price*1.3, 2) where s3.vat_rate=20 and t3.vat_rate=20;
mysql> select * from (t1 as s3, t1 as t3) where s3.vat_rate=20 and t3.vat_rate=20;
1 1.00 1.30 20 1 1.00 1.30 20
2 2.00 1.30 20 1 1.00 1.30 20
1 1.00 1.30 20 2 2.00 1.30 20
2 2.00 1.30 20 2 2.00 1.30 20
If you are trying to set product prices based upon a previous price set then perhaps the following will help clarify things:
mysql> create table t1 (
product_id integer unsigned,
vat_rate integer unsigned,
price decimal(10,2),
price_inc_vat decimal(10,2),
primary key(product_id, vat_rate)
);
mysql> insert into t1 (product_id, price, price_inc_vat, vat_rate) values (1, 1.00, 1.20, 20), (2, 2.00, 2.40, 20), (1, 1.00, 0, 30), (2, 2.00, 0, 30);
mysql> create temporary table tmp1 like t1;
mysql> insert into tmp1 select * from t1;
mysql> select * from t1;
1 20 1.00 1.20
1 30 1.00 0.00
2 20 2.00 2.40
2 30 2.00 0.00
mysql> select * from tmp1;
1 20 1.00 1.20
1 30 1.00 0.00
2 20 2.00 2.40
2 30 2.00 0.00
mysql> update t1 left join tmp1 on t1.product_id=tmp1.product_id and t1.vat_rate > tmp1.vat_rate set t1.price_inc_vat = round(tmp1.price*(1+t1.vat_rate/100), 2) where tmp1.vat_rate = 20;
mysql> select * from t1;
1 20 1.00 1.20
1 30 1.00 1.30
2 20 2.00 2.40
2 30 2.00 2.60

Answer 2

So you have a field for price and another for price_inc_vat and yet another for vat_rate? Why not just store price and vat_rate and calculate price_inc_vat when it's needed?
Also, why are you multiplying by 1.3 when the VAT you're targeting appears to be 20%?
All these logical inconsistencies aside, couldn't you just do the following?
UPDATE products
SET price_inc_vat = ROUND(price * 1.3, 2)
WHERE vat_rate = 20
Or to update everything after a VAT change, disregarding the 10% markup:
UPDATE products
SET price_inc_vat = ROUND(price * (1+vat_rate/100), 2)