Question

CREATE TABLE IF NOT EXISTS `scores` (
  `userID` int(11) NOT NULL,
  `sessionID` int(11) NOT NULL,
  `points` double NOT NULL DEFAULT '0',
  PRIMARY KEY (`userID`,`sessionID`),
  KEY `par_ind1` (`userID`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

What does the line:

KEY `par_ind1` (`userID`)

do? (userID is a primary key in another table?)

Answer 1

KEY is a synonym for INDEX. This creates an index named par_ind1 ont the column userID in addition to the composite key it already shares with sessionID.

See the MySQL CREATE TABLE documentation for the full details, but the relevant part here is:

CREATE [TEMPORARY] TABLE [IF NOT EXISTS] tbl_name
    { LIKE old_tbl_name | (LIKE old_tbl_name) }
create_definition:
    col_name column_definition
  | [CONSTRAINT [symbol]] PRIMARY KEY [index_type] (index_col_name,...)
      [index_option] ...

  /* Key/Index creation */
  | {INDEX|KEY} [index_name] [index_type] (index_col_name,...)
      [index_option] ...

Since no index_type was specified, the default is used. See the CREATE INDEX reference for the default index types, which vary by table storage engine. For an InnoDB table such as this, that’s a BTREE index.