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The CHAR and VARCHAR Column Types
CHAR and VARCHAR are the most commonly used string types. The difference between them is that CHAR is a fixed-length type and VARCHAR is a variable-length type. Values in a CHAR(M) column each take M bytes; shorter values are right-padded with spaces when they are stored. (Trailing spaces are stripped off on retrieval, however.) Values in a VARCHAR(M) column are stored using only as many bytes as necessary, plus one byte to record the length. Trailing spaces are stripped from VARCHAR values when they are stored; this differs from the ANSI SQL standard for VARCHAR values. (A VARCHAR type for which trailing spaces are not stripped may be introduced in a future version of MySQL.)
CHAR and VARCHAR columns can be declared with a maximum length M from 1 to 255. M is optional for CHAR and defaults to 1 if missing. Beginning with MySQL 3.23, CHAR(0) is also legal. CHAR(0) is useful as a placeholder when you want to declare a column but don't want to allocate space for it if you're not sure yet how wide to make it. You can use ALTER TABLE to widen the column later. A CHAR(0) column can also be used to represent on/off values if you allow it to be NULL. Values in such a column can have two values—NULL or the empty string. A CHAR(0) column takes very little storage space in the table—only a single bit. As of MySQL 4.0.2, VARCHAR(0) is allowable as well, but it's treated as CHAR(0).
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