Thoughts on a MySQL statement

Today, I saw a MySQL statement written by my colleagues, but I didn't fully understand it. I suddenly felt the gap. In my mind, MySQL statements only add, delete, query, and modify them, you can still write it like this.

$ SQL = "select category_general_table from product_category where category_id in (select distinct cast (substring (product_code, 1, 5) as unsigned) as CID from product_index where member_id = $ memberid and shop_id = $ shopid )";

I have never written it like this cast or substri ng.

I checked the manual and found another function.ConvertSimilar to it.

Cast (valueType); Convert (value,Type);

Not all types can be dropped. The type that can be converted is as follows:

Binary, with a binary Prefix: Binary
Character type, with parameters: Char ()
Date: Date
Time: Time
Datetime type
Floating Point: decimal
Integer: signed
Unsigned integer

It seems that I still have to read more manuals.