# Placeholder templater

Libraries such as SQLAlchemy or Psycopg use different parameter placeholder styles to mark where a parameter has to be inserted in the query.

For example a query in SQLAlchemy can look like this:

    SELECT * FROM table WHERE id = :myid

At runtime :myid will be replace by a value provided by the application and escaped as needed, but this is not standard SQL and cannot be parsed as is.

In order to parse these queries is then necessary to replace these placeholders with sample values, and this is done with the placeholder templater.

Placeholder templating can be enabled in the config using:

    [sqlfluff]
    templater = placeholder

A few common styles are supported:

     -- colon
     WHERE bla = :my_name

     -- colon_nospaces
     -- (use with caution as more prone to false positives)
     WHERE bla = table:my_name

     -- colon_optional_quotes
     SELECT :"column" FROM :table WHERE bla = :'my_name'

     -- numeric_colon
     WHERE bla = :2

     -- pyformat
     WHERE bla = %(my_name)s

     -- dollar
     WHERE bla = $my_name or WHERE bla = ${my_name}

     -- dollar_surround (DbUp compatible variable)
     WHERE bla = $my_name$

     -- question_mark
     WHERE bla = ?

     -- numeric_dollar
     WHERE bla = $3 or WHERE bla = ${3}

     -- percent
     WHERE bla = %s

     -- ampersand
     WHERE bla = &s or WHERE bla = &{s} or USE DATABASE MARK_{ENV}

These can be configured by setting param_style to the names above:

    [sqlfluff:templater:placeholder]
    param_style = colon
    my_name = 'john'

then you can set sample values for each parameter, like my_name above. Notice that the value needs to be escaped as it will be replaced as a string during parsing. When the sample values aren’t provided, the templater will use parameter names themselves by default.

When parameters are positional, like question_mark, then their name is simply the order in which they appear, starting with 1.

    [sqlfluff:templater:placeholder]
    param_style = question_mark
    1 = 'john'

In case you need a parameter style different from the ones above, you can pass a custom regex.

    [sqlfluff:templater:placeholder]
    param_regex = __(?P<param_name>[\w_]+)__
    my_name = 'john'

N.B. quotes around param_regex in the config are interpreted literally by the templater. e.g. param_regex=’__(?P<param_name>[w_]+)__’ matches ‘__some_param__’ not __some_param__

the named parameter param_name will be used as the key to replace, if missing, the parameter is assumed to be positional and numbers are used instead.

Also consider making a pull request to the project to have your style added, it may be useful to other people and simplify your configuration.