library name: test
table name: goods
Column attribute

 select goods_id, goods_name, (market_price - shop_price) as discount from goods where cat_id!=3; 

Error:

 ERROR 1690 (22003) : a DECIMAL value is out of range in UNSIGNED '(` test `. ` goods `. ` market_price ` - ` test `. ` goods `. ` shop_price `)' 

Why do bosses will appear this kind of circumstance