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I don't understand what an index is or does in SQLite. (NOT SQL) I think it allows for sorting in acending and decending order and access to data quicker. But I'm just guessing here.
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Whenever you create an index, SQLite creates a B-tree structure to hold the index data. The index contains data from the columns that you specify in the index and the corresponding rowid value. This helps SQLite quickly locate the row based on the values of the indexed columns.
An index is a database structure that you can use to improve the performance of database activity. A database table can have one or more indexes associated with it. An index is defined by a field expression that you specify when you create the index. Typically, the field expression is a single field name, like EMP_ID.
In an ordinary SQLite table, the PRIMARY KEY is really just a UNIQUE index. The key used to look up records on disk is the rowid. The special "INTEGER PRIMARY KEY" column type in ordinary SQLite tables causes the column to be an alias for the rowid, and so an INTEGER PRIMARY KEY is a true PRIMARY KEY.
Why not SQL? The answer is the same, though the internal details will differ between implementations.
Putting an index on a column tells the database engine to build, unsurprisingly, an index that allows it to quickly locate rows when you search for certain values in a column, without having to scan every row in the table.
A simple (and probably suboptimal) index might be built with an ordinary binary search tree.
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