> >> First of all, is there any way of limiting the number of rows in a > >> table, referencing to the same element of another table? For example, > >> force a manager not to have more than 10 employees under his control. > >> In a way this can be seen as checking the multiplicity of the > >> relation between the two tables. I know one way would be using > >> triggers, but I was wondering if there was a way of specifying this > >> when the table is constructed. > > In MySQL triggers are the only way. In SQL you might be able to use an > assertion depending on your exact needs. (Don't know if there is any > database that actually implements them per the standard.) > > > > The way to do this would be via CHECK constraints, but MySQL > > doesn't support them. > > CHECK constraints won't work. If I have a parent table and need to > maintain a multiplicity of 1 to 3 children in the child table, how is > a CHECK going to stop somebody from deleting all rows in the child > table?
That depends on your check constraint implementation. Firebird, for example, allows you to reference other tables in SQL statement in your CHECK constraints. So, you could do: exists( select count(*) as cnt from mychildtable where parentid = mytable.parentid and cnt between 1 and 3) (don't know if this syntax is exact, but you get the idea) With regards, Martijn Tonies Database Workbench - tool for InterBase, Firebird, MySQL, Oracle & MS SQL Server Upscene Productions http://www.upscene.com Database development questions? Check the forum! http://www.databasedevelopmentforum.com -- MySQL General Mailing List For list archives: http://lists.mysql.com/mysql To unsubscribe: http://lists.mysql.com/[EMAIL PROTECTED]
