There are also two additional constraints for database and table names, even if you quote them. MySQL's support for names that begin with a number is somewhat unusual among database systems. So we could think in terms of creating a re-usable module which would implement that logic. Referring to Elements of Databases When you use names to refer to elements of databases, you are constrained by the characters you can use and the length that names can be. As of MySQL 3. The separator characters are disallowed in database and table names because databases are represented on disk by directories, and tables are represented on disk by at least one file. MultiRow will just use the base class which implements approaches that should work for any SQL database. There are a few more details worth mentioning. For the first edition of this book, the list of missing features included transactions, subselects, foreign keys, and referential integrity. Even if we wanted to update a thousand rows with different values, we could still do it with four statements. Aliases can be fairly arbitrary, but you should quote an alias within single or double quotes if it is a SQL keyword, is entirely numeric, or contains spaces or other special characters. Quoting a name also allows it to be entirely numeric, something that is not true of unquoted names. Similarly, if a query uses multiple tables and refers to a column name that is present in more than one table, it's necessary to qualify the name with a table name to make it clear which column you mean. If you're using a SELECT statement that refers to only one table, that table is implicit for every column reference in the statement. Does it mean the same thing, or is it a number in scientific notation? You should consider lettercase issues if you create a database on a server with case-sensitive filenames and you might someday move the database to a server where filenames are not case sensitive.
One way to avoid having case sensitivity properties become an issue is to pick a given lettercase for example, lowercase and always create databases and tables using names in that lettercase. This is in fact entirely possible in many database systems. This is useful when a name contains special characters or is a reserved word. MultiRow, we would write: The following two queries refer to the same pair of column names, but the context supplied by the FROM clause of each statement indicates from which table to select the columns: Databases and tables in MySQL are implemented using directories and files in the underlying file system on the server host. The following queries are equivalent: Column and index names are not case sensitive in MySQL. To refer to a database, just specify its name: It's necessary to qualify names only when a table or database cannot be determined from context. Names for databases, tables, columns, and indexes can be up to 64 characters long. John and we could persuade the database server to apply those updates to the target table? They can be given in any lettercase. That is not true for SQL keywords, function names, or column and index names, all of which can be referred to in varying lettercase style throughout a query. It is relatively straightforward to populate a table with multiple rows with just one query or at least, far fewer queries than the number of rows desired. At the time of writing, the only database-specific subclass is for PostgreSQL. Referring to Elements of Databases When you use names to refer to elements of databases, you are constrained by the characters you can use and the length that names can be. The separator characters are disallowed in database and table names because databases are represented on disk by directories, and tables are represented on disk by at least one file. SQL keywords and function names. That's probably because for many or most applications, those features don't matter. There are a few more details worth mentioning. An indication of which row should be updated New values for one or more fields Going back to our first example: However, a name cannot consist entirely of digits because that would make it indistinguishable from a number. Does it mean the same thing, or is it a number in scientific notation? Consequently, these types of names must not contain characters that are illegal in directory names and filenames.
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