How Do I Enter the Current Date and Time into a Field
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When inserting records into a table containing a TIMESTAMP field, MySQL automatically fills that field with the current date and time if no other value was specified To accomplish the same thing with other date/time fields, use the NOW() function Finally, for simple applications that only need to store the year, MySQL offers the special YEAR type, which accepts a four-digit year value It s worthwhile to use this value if your application deals mostly with the year component of a date value, because a field marked as YEAR occupies 1 byte on disk (as compared to a DATETIME or DATE field, which can occupy up to 8 bytes) MySQL YEAR fields can accept any value in the range 1901 to 2155
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For situations where a field value must be selected from a predefined list of values, MySQL offers the ENUM and SET data types For both these types, a list of predefined values must be included as part of the type definition An ENUM field definition can contain up to 65,536 elements, while a SET field definition can hold up to 64 elements For a field marked as an ENUM field, only one of the predefined values may be selected, whereas for a field marked as a SET field, zero, one, or more than one of the pre-defined values may be selected ENUM fields are best suited for mutually exclusive values, while SET fields are best suited for independent values As an example, the definition ENUM ( red , green , yellow ) forces entry of any one of the three values, while the definition SET ( mon , tue , wed , thu , fri ) allows entry of none, one, or all of the five values In addition, SET values are stored as bits, making it possible to perform bitwise comparison and sorting operations on them
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What Happens if I Try Inserting an Unlisted Value into an ENUM or SET Field
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With both ENUM and SET types, attempting to insert a value that does not exist in the predefined list of values will cause MySQL to insert either an empty string or a 0
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To decide the data type for a field, take into account the following factors: The range and type of values that the field will hold The types of calculations you expect to perform on those values The manner in which the data is to be formatted for display purposes
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The manner in which the data is to be sorted and compared against other fields The available subtypes for each field and their storage efficiencies
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By taking all of these factors into consideration when designing your database, you reduce the chance of incompatibilities and storage inefficiencies later
Selecting Table Storage Engines
As Table 2-2 in 2 illustrates, MySQL supports many different storage engines for its tables, each with its own advantages and disadvantages While all of MySQL s storage engines are reasonably efficient, using the wrong storage engine can hinder your application from achieving its maximum possible performance For example, using the ARCHIVE engine for a table that will see frequent reads and writes will produce significantly slower performance than using the MYISAM engine for the same table To help you choose the most appropriate engine for your table, the following sections discuss each of these engines in greater detail
The MyISAM Storage Engine
The MyISAM storage engine extends the base ISAM type with a number of additional optimizations and enhancements, and is MySQL s default table type MyISAM tables are optimized for compression and speed, and are immediately portable between different OSs and platforms (for example, the same MyISAM table can be used on both Windows and UNIX OSs) The MyISAM format supports large table files (up to 256TB in size) and allows indexing of BLOB and TEXT columns Tables and table indexes can be compressed to save space, a feature that comes in handy when storing large BLOB or TEXT fields VARCHAR fields can either be constrained to a specific length or adjusted dynamically as per the data within them, and the format supports searching for records using any key prefix, as well as using the entire key Because MyISAM tables are optimized for MySQL, it s no surprise that the developers added a fair amount of intelligence to them MyISAM tables can be either fixed-length or dynamic-length MySQL automatically checks MyISAM tables for corruption on startup and can even repair them in case of errors Table data and table index files can be stored in different locations, or even on different file systems And intelligent defragmentation logic ensures a high-performance coefficient, even for tables with a large number of inserts, updates, and deletions Large MyISAM tables can also be compressed, or packed, into smaller read-only tables that take up less disk space, with MySQL s myisampack utility