SQL Constructor

A programming approach (and supporting functions) to programmatically construct and maintain well-formatted SQL from Python 3.6 and above.

Motivation and Philosophy

Like all programming languages, the task of writing maintainable SQL is made a little bit easier through good formatting and layout, reducing the cognitive load on the programmers. Doing this with completely static SQL is possible e.g. with static formatters, but is harder to achieve when programmatically constructing the SQL. One can easily loose the formating in both the generating Python code and in the generated SQL. Ouch.

This package aims to solve this problem with a particular programming pattern and a small number of utility functions.

In particular, the objectives are:

• To have the SQL look as much like SQL within the source Python code whilst still using obvious Python to generate it. This involves trade-offs.
• To have the generated SQL look like it could have been written directly so that it can be read and understood as easily as possible.
• To be able to write optimum SQL in SQL rather than trying to write optimum SQL from another language but mentally thinking in the target SQL.

It should be clear that SQL Constructor is not an ORM. There is no abstraction. In fact, quite the opposite.

SQL variants

This library has been used to generate SQL for PostgreSQL. However it should work with other database engines / language variants with little or no change. Similarly, it is agnostic to SQL code style conventions because that is controlled by the programmer.

In fact the whole idea is not really about SQL at all, but about managing text.

Example

A simple example will illustrate the approach. From the perspective of SQL the subquery is unnecessary; it is used purely to show composition.

import sqlcon

sq = sqlcon.single_quote
dq = sqlcon.double_quote


def select_columns(variables):
    yield sqlcon.joinwith(dq(v) for v in variables)


def subquery():
    yield """
        SELECT
            *
        FROM
            some_table
        LEFT JOIN
            some_other_table
        USING
            some_table.id = some_other_table.key
    """, -1


def where_clauses(variables, conditions):
    for condition in conditions:
        variable, comparator, constant = condition
        assert variable in variables, f"Unknown variable: {variable}"
        assert comparator in ("=", "~"), f"Unknown comparator: {comparator}"
        yield f"{dq(variable)} {comparator} {sq(constant)}"


def example(variables, conditions):
    yield """
        SELECT
    """
    yield 1, select_columns(variables), -1
    yield """
        FROM
            (
    """
    yield 1, subquery(), -1
    yield """
            ) AS tmp
        WHERE
    """
    yield sqlcon.indented_joinwith(
        where_clauses(variables, conditions), separator=" AND "
    )


if __name__ == "__main__":
    sql = example(
        ["name", "age", "address"],
        [("name", "=", "tim"), ("address", "~", "England")],
    )
    print(sqlcon.process(sql))

When run, this produces:

SELECT
    "name",
    "age",
    "address"
FROM
    (
        SELECT
            *
        FROM
            some_table
        LEFT JOIN
            some_other_table
        USING
            some_table.id = some_other_table.key
    ) AS tmp
WHERE
    "name" = 'tim' AND
    "address" ~ 'England'

The processing takes strings (for the actual SQL), integers (for manual indentation changes), and lists/tuples/generators for composition of the above. Clearly this example is rather degenerate. It also mixes a few styles which is inconsistent but illustrates a few different approaches.

Note how some relatively subtle things are happening automatically:

1. Common indentation is being removed to left align the base of the generated SQL.
2. Blank lines are being stripped intelligently. E.g. the start and end of the tripple quoted strings.
3. The indentation levels are being tracked. E.g. the subquery is indented in the output but not in the subquery() function. So nested layers (such as views within views or views within PostgreSQL functions) can be written neatly without worrying about the indentation of their containing scope.

API

The SQL Constructor API consists of a small number of functions. The primary function is process; the others are auxilliary.

• process will convert the SQL strings, indentation integers, and composition of these as iterables all into output SQL.
• single_quote will correctly quote literal strings for PostgreSQL.
• double_quote will correctly quote identifiers for PostgresSQL.
• joinwith will join lists with commas (e.g. for lists of column identifers or "IN" clauses) or conditions with "AND" (e.g. for conjunctions in WHERE clauses). Can be used either on a single line or across multiple lines with maintained indentation.
• indented_joinwith provides a shorthand to prefix joinwth with an indent and postfix with an outdent.

See the source code docstrings for details.

Tests

See the unittests in the tests/ directory.

Alternatives

Before deciding to create SQL Constructor I tried various approaches "in anger" on real projects. Most notably:

• Templating the SQL either with vanilla Jinja2 or variants designed to work with SQL like JinjaSQL (e.g. https://github.com/hashedin/jinjasql). Maintaining formatting is difficult, and now you are working in 2 files (Python and the template) and 3 languages (templating, Python, and SQL).

• Using an ORM like SQLAlchemy. Whilst superficially clean ("it's all just Python"), anything more than the most trivial quickly requires mental gymnastics thinking in SQL but writing in Python, and needing to drop out of the Python API to create separate SQL support functions and views etc in order to get the most from the database engine.

Neither of these worked for me.

Contributions

Please share other approaches you have tried. Perhaps there is a better way?! Similarly, pull-requests and bug reports etc are all welcome.

Release checklist

• Run: black .
• Run: isort -y
• Run: flake8 .
• Run: nose2 -v --with-coverage tests
• Run: poetry export -f requirements.txt >requirements.txt (for snyk scanning)
• Ensure git tag, the package version (via poetry version), and sqlcon.__version__ are all equal!