When something ISN’T AS GOOD (PRETTY, FAST..) AS something else, we use 没有 /méiyǒu/ instead of 比 /bǐ/
EXAMPLES
Beijing isn't as safe as you think.
This isn’t as good as that.
这个 没有 那个 好。
Zhège méiyǒu nàge hǎo.
Chinese food isn’t as good as Japanese food.
中国菜 没有 日本菜 好吃。
Zhōngguó cài méiyǒu Rìběn cài hào chī.
Chinese isn't as difficult as Japanese.
My Chinese isn't as fluent as his.
Her hair isn't as long as before.
My Chinese isn't as good as his.
我的 中文 没有 他 (的) 好。
<Wǒde Zhōngwén méiyǒu tā hǎo.>
Their relationship isn't as good as before.