Release: 1.2.12 current release | Release Date: September 19, 2018

# SQL Expressions¶

## How do I render SQL expressions as strings, possibly with bound parameters inlined?¶

The “stringification” of a SQLAlchemy statement or Query in the vast majority of cases is as simple as:

print(str(statement))

this applies both to an ORM Query as well as any select() or other statement. Additionally, to get the statement as compiled to a specific dialect or engine, if the statement itself is not already bound to one you can pass this in to ClauseElement.compile():

print(statement.compile(someengine))

or without an Engine:

from sqlalchemy.dialects import postgresql
print(statement.compile(dialect=postgresql.dialect()))

When given an ORM Query object, in order to get at the ClauseElement.compile() method we only need access the statement accessor first:

statement = query.statement
print(statement.compile(someengine))

The above forms will render the SQL statement as it is passed to the Python DBAPI, which includes that bound parameters are not rendered inline. SQLAlchemy normally does not stringify bound parameters, as this is handled appropriately by the Python DBAPI, not to mention bypassing bound parameters is probably the most widely exploited security hole in modern web applications. SQLAlchemy has limited ability to do this stringification in certain circumstances such as that of emitting DDL. In order to access this functionality one can use the literal_binds flag, passed to compile_kwargs:

from sqlalchemy.sql import table, column, select

t = table('t', column('x'))

s = select([t]).where(t.c.x == 5)

print(s.compile(compile_kwargs={"literal_binds": True}))

the above approach has the caveats that it is only supported for basic types, such as ints and strings, and furthermore if a bindparam() without a pre-set value is used directly, it won’t be able to stringify that either.

To support inline literal rendering for types not supported, implement a TypeDecorator for the target type which includes a TypeDecorator.process_literal_param() method:

from sqlalchemy import TypeDecorator, Integer

class MyFancyType(TypeDecorator):
impl = Integer

def process_literal_param(self, value, dialect):
return "my_fancy_formatting(%s)" % value

from sqlalchemy import Table, Column, MetaData

tab = Table('mytable', MetaData(), Column('x', MyFancyType()))

print(
tab.select().where(tab.c.x > 5).compile(
compile_kwargs={"literal_binds": True})
)

producing output like:

SELECT mytable.x
FROM mytable
WHERE mytable.x > my_fancy_formatting(5)

## I’m using op() to generate a custom operator and my parenthesis are not coming out correctly¶

The Operators.op() method allows one to create a custom database operator otherwise not known by SQLAlchemy:

>>> print(column('q').op('->')(column('p')))
q -> p

However, when using it on the right side of a compound expression, it doesn’t generate parenthesis as we expect:

>>> print((column('q1') + column('q2')).op('->')(column('p')))
q1 + q2 -> p

Where above, we probably want (q1 + q2) -> p.

The solution to this case is to set the precedence of the operator, using the Operators.op.precedence parameter, to a high number, where 100 is the maximum value, and the highest number used by any SQLAlchemy operator is currently 15:

>>> print((column('q1') + column('q2')).op('->', precedence=100)(column('p')))
(q1 + q2) -> p

We can also usually force parenthesization around a binary expression (e.g. an expression that has left/right operands and an operator) using the ColumnElement.self_group() method:

>>> print((column('q1') + column('q2')).self_group().op('->')(column('p')))
(q1 + q2) -> p

### Why are the parentheses rules like this?¶

A lot of databases barf when there are excessive parenthesis or when parenthesis are in unusual places they doesn’t expect, so SQLAlchemy does not generate parenthesis based on groupings, it uses operator precedence and if the operator is known to be associative, so that parenthesis are generated minimally. Otherwise, an expression like:

column('a') & column('b') & column('c') & column('d')

would produce:

(((a AND b) AND c) AND d)

which is fine but would probably annoy people (and be reported as a bug). In other cases, it leads to things that are more likely to confuse databases or at the very least readability, such as:

column('q', ARRAY(Integer, dimensions=2))[5][6]

would produce:

((q[5])[6])

There are also some edge cases where we get things like "(x) = 7" and databases really don’t like that either. So parenthesization doesn’t naively parenthesize, it uses operator precedence and associativity to determine groupings.

For Operators.op(), the value of precedence defaults to zero.

What if we defaulted the value of Operators.op.precedence to 100, e.g. the highest? Then this expression makes more parenthesis, but is otherwise OK, that is, these two are equivalent:

>>> print (column('q') - column('y')).op('+', precedence=100)(column('z'))
(q - y) + z
>>> print (column('q') - column('y')).op('+')(column('z'))
q - y + z

but these two are not:

>>> print column('q') - column('y').op('+', precedence=100)(column('z'))
q - y + z
>>> print column('q') - column('y').op('+')(column('z'))
q - (y + z)

For now, it’s not clear that as long as we are doing parenthesization based on operator precedence and associativity, if there is really a way to parenthesize automatically for a generic operator with no precedence given that is going to work in all cases, because sometimes you want a custom op to have a lower precedence than the other operators and sometimes you want it to be higher.

It is possible that maybe if the “binary” expression above forced the use of the self_group() method when op() is called, making the assumption that a compound expression on the left side can always be parenthesized harmlessly. Perhaps this change can be made at some point, however for the time being keeping the parenthesization rules more internally consistent seems to be the safer approach.

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