xref: /external/chromium_org/third_party/sqlite/src/ext/fts3

1. OVERVIEW

  This README file describes the syntax of the arguments that may be passed to
  the FTS3 MATCH operator used for full-text queries. For example, if table 
  "t1" is an Fts3 virtual table, the following SQL query:

    SELECT * FROM t1 WHERE <col> MATCH <full-text query>

  may be used to retrieve all rows that match a specified for full-text query. 
  The text "<col>" should be replaced by either the name of the fts3 table 
  (in this case "t1"), or by the name of one of the columns of the fts3 
  table. <full-text-query> should be replaced by an SQL expression that 
  computes to a string containing an Fts3 query.

  If the left-hand-side of the MATCH operator is set to the name of the
  fts3 table, then by default the query may be matched against any column
  of the table. If it is set to a column name, then by default the query
  may only match the specified column. In both cases this may be overriden
  as part of the query text (see sections 2 and 3 below).

  As of SQLite version 3.6.8, Fts3 supports two slightly different query 
  formats; the standard syntax, which is used by default, and the enhanced
  query syntax which can be selected by compiling with the pre-processor
  symbol SQLITE_ENABLE_FTS3_PARENTHESIS defined.

    -DSQLITE_ENABLE_FTS3_PARENTHESIS

2. STANDARD QUERY SYNTAX

  When using the standard Fts3 query syntax, a query usually consists of a 
  list of terms (words) separated by white-space characters. To match a
  query, a row (or column) of an Fts3 table must contain each of the specified
  terms. For example, the following query:

    <col> MATCH 'hello world'

  matches rows (or columns, if <col> is the name of a column name) that 
  contain at least one instance of the token "hello", and at least one 
  instance of the token "world". Tokens may be grouped into phrases using
  quotation marks. In this case, a matching row or column must contain each
  of the tokens in the phrase in the order specified, with no intervening
  tokens. For example, the query:

    <col> MATCH '"hello world" joe"

  matches the first of the following two documents, but not the second or
  third:

    "'Hello world', said Joe."
    "One should always greet the world with a cheery hello, thought Joe."
    "How many hello world programs could their be?"

  As well as grouping tokens together by phrase, the binary NEAR operator 
  may be used to search for rows that contain two or more specified tokens 
  or phrases within a specified proximity of each other. The NEAR operator
  must always be specified in upper case. The word "near" in lower or mixed
  case is treated as an ordinary token. For example, the following query:

    <col> MATCH 'engineering NEAR consultancy'

  matches rows that contain both the "engineering" and "consultancy" tokens
  in the same column with not more than 10 other words between them. It does
  not matter which of the two terms occurs first in the document, only that
  they be seperated by only 10 tokens or less. The user may also specify
  a different required proximity by adding "/N" immediately after the NEAR
  operator, where N is an integer. For example:

    <col> MATCH 'engineering NEAR/5 consultancy'

  searches for a row containing an instance of each specified token seperated
  by not more than 5 other tokens. More than one NEAR operator can be used
  in as sequence. For example this query:

    <col> MATCH 'reliable NEAR/2 engineering NEAR/5 consultancy'

  searches for a row that contains an instance of the token "reliable" 
  seperated by not more than two tokens from an instance of "engineering",
  which is in turn separated by not more than 5 other tokens from an
  instance of the term "consultancy". Phrases enclosed in quotes may
  also be used as arguments to the NEAR operator.

  Similar to the NEAR operator, one or more tokens or phrases may be 
  separated by OR operators. In this case, only one of the specified tokens
  or phrases must appear in the document. For example, the query:

    <col> MATCH 'hello OR world'

  matches rows that contain either the term "hello", or the term "world",
  or both. Note that unlike in many programming languages, the OR operator
  has a higher precedence than the AND operators implied between white-space
  separated tokens. The following query matches documents that contain the
  term 'sqlite' and at least one of the terms 'fantastic' or 'impressive',
  not those that contain both 'sqlite' and 'fantastic' or 'impressive':

    <col> MATCH 'sqlite fantastic OR impressive'

  Any token that is part of an Fts3 query expression, whether or not it is
  part of a phrase enclosed in quotes, may have a '*' character appended to
  it. In this case, the token matches all terms that begin with the characters
  of the token, not just those that exactly match it. For example, the 
  following query:

    <col> MATCH 'sql*'

  matches all rows that contain the term "SQLite", as well as those that
  contain "SQL".

  A token that is not part of a quoted phrase may be preceded by a '-'
  character, which indicates that matching rows must not contain the 
  specified term. For example, the following:

    <col> MATCH '"database engine" -sqlite'

  matches rows that contain the phrase "database engine" but do not contain
  the term "sqlite". If the '-' character occurs inside a quoted phrase,
  it is ignored. It is possible to use both the '-' prefix and the '*' postfix
  on a single term. At this time, all Fts3 queries must contain at least
  one term or phrase that is not preceded by the '-' prefix.

  Regardless of whether or not a table name or column name is used on the 
  left hand side of the MATCH operator, a specific column of the fts3 table
  may be associated with each token in a query by preceding a token with
  a column name followed by a ':' character. For example, regardless of what
  is specified for <col>, the following query requires that column "col1"
  of the table contains the term "hello", and that column "col2" of the
  table contains the term "world". If the table does not contain columns
  named "col1" and "col2", then an error is returned and the query is
  not run.

    <col> MATCH 'col1:hello col2:world'

  It is not possible to associate a specific table column with a quoted 
  phrase or a term preceded by a '-' operator. A '*' character may be
  appended to a term associated with a specific column for prefix matching.

3. ENHANCED QUERY SYNTAX

  The enhanced query syntax is quite similar to the standard query syntax,
  with the following four differences:

  1) Parenthesis are supported. When using the enhanced query syntax,
     parenthesis may be used to overcome the built-in precedence of the
     supplied binary operators. For example, the following query:

       <col> MATCH '(hello world) OR (simple example)'

     matches documents that contain both "hello" and "world", and documents
     that contain both "simple" and "example". It is not possible to forumlate
     such a query using the standard syntax.

  2) Instead of separating tokens and phrases by whitespace, an AND operator
     may be explicitly specified. This does not change query processing at
     all, but may be used to improve readability. For example, the following
     query is handled identically to the one above:

       <col> MATCH '(hello AND world) OR (simple AND example)'

     As with the OR and NEAR operators, the AND operator must be specified
     in upper case. The word "and" specified in lower or mixed case is 
     handled as a regular token.

  3) The '-' token prefix is not supported. Instead, a new binary operator,
     NOT, is included. The NOT operator requires that the query specified
     as its left-hand operator matches, but that the query specified as the
     right-hand operator does not. For example, to query for all rows that
     contain the term "example" but not the term "simple", the following
     query could be used:

       <col> MATCH 'example NOT simple'

     As for all other operators, the NOT operator must be specified in
     upper case. Otherwise it will be treated as a regular token.

  4) Unlike in the standard syntax, where the OR operator has a higher
     precedence than the implicit AND operator, when using the enhanced
     syntax implicit and explict AND operators have a higher precedence
     than OR operators. Using the enhanced syntax, the following two
     queries are equivalent:

       <col> MATCH 'sqlite fantastic OR impressive'
       <col> MATCH '(sqlite AND fantastic) OR impressive'

     however, when using the standard syntax, the query:

       <col> MATCH 'sqlite fantastic OR impressive'

     is equivalent to the enhanced syntax query:

       <col> MATCH 'sqlite AND (fantastic OR impressive)'

     The precedence of all enhanced syntax operators, in order from highest
     to lowest, is:

       NEAR       (highest precedence, tightest grouping)
       NOT
       AND
       OR         (lowest precedence, loosest grouping)

  Using the advanced syntax, it is possible to specify expressions enclosed
  in parenthesis as operands to the NOT, AND and OR operators. However both
  the left and right hand side operands of NEAR operators must be either
  tokens or phrases. Attempting the following query will return an error:

    <col> MATCH 'sqlite NEAR (fantastic OR impressive)'

  Queries of this form must be re-written as:

    <col> MATCH 'sqlite NEAR fantastic OR sqlite NEAR impressive'