Two questions (combining UPDATE queries and free up locks)

I am hoping this reaches the right place, I was sure I had sent it but didn't see it come to list... I have two problems I am hoping to solve. 1) Is it possiblt to "combine" two update queries into one. So something like UPDATE table SET field=value WHERE field=value2 which makes up one stateme

Table Locking - Max Number of Users at Once?

I have always used flat file text files for my database purposes but this has always led to problems when the number of users gets to be too many. I am now using mySQL and want to know how many people can use the db at the same time without causing problems. As for table locking, should it al