What each column can hold.
NUMBERS
INT for whole numbers, with variants for different ranges DECIMAL for exact values, which is what money should use FLOAT and DOUBLE for approximate values, which money should never use
Storing prices as floating point produces rounding errors that appear in totals.
TEXT
VARCHAR for short text with a defined maximum TEXT for longer content LONGTEXT for very long content, such as post bodies
A VARCHAR limited to 255 characters rejects anything longer, producing a data too long error.
DATES
DATE for a date alone DATETIME for a date and time TIMESTAMP, which is stored relative to a timezone and converted on retrieval
The distinction between DATETIME and TIMESTAMP causes genuine confusion when timezones differ.
OTHER
BOOLEAN, usually stored as a small integer ENUM, a fixed list of permitted values JSON, in newer versions
WHY THIS MATTERS
Inserting a value the column cannot hold produces an error. Understanding the type explains why.
