Ever since I have released Slonik (PostgreSQL client for Node.js) and written a controversial Stop using Knex.js article (tl;dr; query builders are designed to be building blocks for ORMs; they do not add value when majority of the query is static.), I have been asked a lot – then how do I generate dynamic queries? I will answer this by sharing a couple of real-life examples.
All of the queries in this article are actual queries used in real-life business, Applaudience which heavily relies on PostgreSQL.
Disclaimer: (1) All examples discuss only SQL injection threats. Authorization logic (e.g. whitelisting columns user is authorized to access) is not in the scope of this article. (2) All statements assume there are no bugs in Slonik implementation.
Static query with dynamic value bindings
If your query logic does not change depending on user's input, then simply construct SQL query using sql
tagged template literal, e.g.
sql`
SELECT c1.country_id
FROM cinema_movie_name cmn1
INNER JOIN cinema c1 ON c1.id = cmn1.cinema_id
WHERE cmn1.id = ${cinemaMovieNameId}
`;
If you are using Slonik, it is safe to pass values as template literal placeholders. sql
will interpret all placeholder tokens and construct final SQL query. In this case, the only dynamic part of the query is the value bindings themselves, therefore the final query is:
SELECT c1.country_id
FROM cinema_movie_name cmn1
INNER JOIN cinema c1 ON c1.id = cmn1.cinema_id
WHERE cmn1.id = $1
Query and bound values will be sent to PostgreSQL separately: no risk of SQL injection.
Binding a list of values
When your query input is a list of values (e.g. such as when retrieving rows matching multiple identifiers), then you may use sql.valueList
, e.g.
sql`
SELECT m1.*
FROM movie m1
WHERE m1.id IN (${sql.valueList(movieIds)})
`;
This will generate a query with dynamic set of value bindings, i.e. if movieIds
is [1, 2, 3]
the query that is sent to PostgreSQL will be:
SELECT m1.*
FROM movie m1
WHERE m1.id IN ($1, $2, $3)
However, despite this being a common pattern, I do not advise to use this pattern. Instead, use sql.array
, e.g.
sql`
SELECT m1.*
FROM movie m1
WHERE m1.id = ANY(${sql.array(movieIds, 'int4')})
`;
This will generate a fixed-length query that does not change based on its inputs, i.e.
SELECT m1.*
FROM movie m1
WHERE m1.id = ANY($1::"int4"[])
Continue reading sql.array
vs sql.valueList
.
Query with dynamic columns
If your query result refers to columns that depend on user's input, then use sql.identifier
to generate SQL that identifies those columns, e.g.
(Note: Not an actual query used in business. See next paragraph.)
sql`
SELECT m1.id, ${sql.identifier(['m1', movieTableColumnName])}
FROM movie m1
WHERE
m1.id = ${moveId}
`;
This query will produce a query that selects exactly 1 dynamically identified column. There is no risk of SQL injection, i.e. even if logic leading to generation of movieTableColumnName
was somehow compromised, the worst that can happen is that query attacker will be able to return any column under m1
alias or execute query with invalid column identifier values (both carry risk; business logic is not in scope of this article).
Just because you can do this, you probably shouldn't. When your application requires to return different columns depending on user's query, it is better to select all columns that are in scope of the business logic and pick value of the needed column, i.e. If the intent of the latter query was to return a different movie identifier based on movieTableColumnName
, then it is better to write a static query:
sql`
SELECT
m1.id,
m1.foreign_comscore_id,
m1.foreign_imdb_id,
m1.foreign_metacritic_id
m1.foreign_rottentomatoes_id,
m1.foreign_tmdb_id,
m1.foreign_webedia_id
FROM movie m1
WHERE
m1.id = ${moveId}
`;
The latter has does return some superfluous data on every query, but it has several advantages:
- It reduces risk of SQL injection (regardless of how much you trust code generation logic, static code is always safer than dynamic code).
- It produces only one entry
pg_stat_statements
. You will learn to appreciate as few as possible queries inpg_stat_statements
as your application scales.
Query with multiple dynamic columns
Same as the above, but sql.identifierList
.
Nesting dynamic SQL queries
sql
tagged template literals can be nested, e.g.
(Note: Simplified version of an actual query used in business.)
const futureEventEventChangeSqlToken = sql`
SELECT
ec1.event_id,
ec1.seat_count,
ec1.seat_sold_count
FROM event_change_future_event_view ec1
`;
sql`
SELECT
event_id,
seat_count,
seat_sold_count
FROM (
${futureEventEventChangeSqlToken}
) AS haystack
WHERE ${paginatedWhereSqlToken}
ORDER BY ${orderSqlToken}
LIMIT ${limitSqlToken}
`
This allows to pass pre-bound SQL queries as first-class citizens across your program. This is handy when the intent is to isolate SQL generation logic for testing or when large SQL fragments are shared between queries or when the intent is to simply reduce concentration of code complexity in one place.
Injecting dynamic SQL fragments
sql.raw
is used to inject dynamic SQL fragments, i.e.
sql`
SELECT ${sql.raw('foo bar baz')}
`
translates to (invalid) query:
SELECT foo bar baz
Unlike the previous example using sql
tagged template, sql.raw
is not safe – it allows to create dynamic SQL using user input.
There are no known use cases for generating queries using sql.raw
that aren't covered by nesting bound sql
expressions (described in "Nesting dynamic SQL queries") or by one of the other existing query building methods. sql.raw
exists as a mechanism to execute externally stored static (e.g. queries stored in files).
Query with a dynamic comparison predicate members or operator
If an operator of a comparison predicate present in your query is dynamic, then use sql.comparisonPredicate
, e.g.
(Note: Not an actual query used in business.)
sql`
SELECT
c1.id,
c1.nid,
c1.name
FROM cinema c1
WHERE
${sql.comparisonPredicate(
sql`c1.name`,
nameComparisonOperator,
nameComparisonValue
)}
`;
nameComparisonOperator
can be values such as =
, >
, <
, etc. Assuming nameComparisonOperator
is "=", then the resulting query is going to be:
SELECT
c1.id,
c1.nid,
c1.name
FROM cinema c1
WHERE
c1.name = $1
The latter is an extremely rare use case, reserved almost entirely to building higher level SQL abstraction tools (such as ORMs). It may be useful for "advance search" scenarios, however continue reading to familiarise with alternative patterns (see sql.booleanExpression
).
Query with dynamic WHERE clause members
If presence of WHERE
clause members is dynamic, then use sql.booleanExpression
.
const findCinemas = (root, parameters, context) => {
const booleanExpressions = [
sql`TRUE`,
];
if (parameters.input.query) {
const query = parameters.input.query;
if (query.countryId !== undefined) {
booleanExpressions.push(
sql`c2.id = ${query.countryId}`
);
}
if (query.nid !== undefined) {
booleanExpressions.push(
sql`c1.nid % ${query.nid}`
);
}
if (query.name !== undefined) {
booleanExpressions.push(
sql`c1.name % ${query.name}`
);
}
}
const whereSqlToken = sql.booleanExpression(
booleanExpressions,
'AND'
);
return context.pool.any(sql`
SELECT
c1.id,
c1.nid,
c1.name,
c2.code_alpha_2 country_code,
c2.name country_name
FROM cinema c1
INNER JOIN country c2 ON c2.id = c1.country_id
WHERE ${whereSqlToken}
`);
},
findCinemas
is an implementation of a GraphQL resolver. WHERE clause of the query is constructed using a combination of 3 possible boolean expressions. As is the case with all the other query building methods in Slonik, all expressions can be nested: you can have other boolean expressions as members of a boolean expression or even SQL expression constructed using sql
tagged template literal.
Summary
These examples cover every common dynamic SQL building scenario and provide enough knowledge of how Slonik works to enable reader to continue journey of familiarising with other query building methods provided by Slonik. The primary intent of this article was to demonstrate that Slonik provides a safe abstraction for constructing SQL queries keeping the static parts of the query intact.
If you value my work and want to see Slonik and many other of my Open-Source projects to be continuously improved, then please consider becoming a patron:
Finally, I missed a use case scenario that you would like me to cover, mention it in the comments and I will happily include it.
Top comments (26)
Is it ok to use
sql.raw
to create a fully dynamicORDER BY
query?You don't need to:
sql
tags can be nested.Thanks. What about optional
ORDER BY
? ForWHERE
you started withTRUE
so the absence of conditions results inWHERE (true)
which is syntactically correct and has no performance implications. I can't find anything than can be used to order by default:Another attempt with
?:
breaks the placeholder ordering in query:Causes
I cannot recall the original reason for this constraint. Will likely remove this constraint. Track github.com/gajus/slonik/issues/93.
Another question. Why
valueList
and other similar functions support only primitive values?Is TypeScript a reason? The thing I like about
pg
driver is that, unlike MySQL, I canSELECT
orINSERT
a date, a boolean, an object, etc. value an it will do "the right thing" without an extra hassle.Maybe it's not entirely safe or somehow "not a good practice" – I dunno.
For now, the API feels limiting for no obvious reason.
Value token can be any primitive that is shared between pg and JavaScript (string, integer, float, boolean, null).
If you have a requirement for automatic coalescing of other object types (such as Date), I suggest raising an issue. I cannot think of other types that would be relatively safe to cast, though.
JS Object with
JSON.stringify
?Too many edges cases where an object could be passed accidentally inserting potentially sensitive data to the database.
There is
sql.json()
for that, though.Slonik's approach is very interesting. Although it's maybe not directly related to Slonik, but because we don't use an ORM: What would be the best way for transforming a SELECT query result of rows into an array of nested objects? For instance if we are joining multiple tables and have a master-detail-relationship. The web client usually expects some kind of nested JSON structure.
You can return JSON from PostgreSQL query.
So your recommendation is to do everything in PostgreSQL with its JSON functions? So we already have the ready-to-use object or array of objects as query result?
Not at all. I think it is entirely unnecessary. Keep the queries simple. If you need to nest objects, either use frameworks that automate nesting (e.g. GraphQL), or write simple iteration routines to amend the data structures.
Sounds good, thank you! Do you know of any application which uses Slonik + GraphQL and is open source, so that I can study this approach?
Seems that
sql.valueList
does not exist anymore. Also the link above providedContinue reading sql.array vs sql.valueList.
does not lead to the right section of the README in the repo.Was this feature removed lately?
sql.booleanExpression
was removed in favour ofsql.join
but the article has not been updated.See feat: remove multiple methods in favor of sql.join
The doc link is now github.com/gajus/slonik#slonik-que...
Hello Gajus,
I've been experimenting with Slonik for the first time today. Seems interesting, I'm very much aligned with the philosophy of "just write plain old sql". This article was useful, especially the final example. Thanks.
What's the difference between
tuple
andvalueList
? The only one I see is thattuple
adds(
and)
parens around its values... The implementation and types are almost identical tovalueList
, to the point I wonder why not to express one in terms of another or just ditchtuple
as unnecessary. Most probably I miss something – that's just my first impression.Eventually
sql.valueList
,sql.rawList
,sql.tupleList
,sql.identifierList
andsql.booleanExpression
will be removed in favour of a single methodsql.expressionList
.github.com/gajus/slonik/issues/91
The original intention for separation was to force semantic resemblance and type strictness specific to the code fragment being generated. However, since then types have evolved for all of these helpers to allow a lot broader spectrum of allowed values.
Where have all the sql builder functions go and how to replace them?
comparisonPredicate, valueList, etc.
I am trying to construct multiple WHERE statements dynamically, but I get all sorts of errors. The thing is, some of them might be undefined and thus ommited, but then sql builder yells that it's not a valid value
Hey Garjus - this is the first time I'm coming across your articles/project. Looks really interesting! I look foward to digging in :) I would love to hear your thoughts on github.com/craigmichaelmartin/pure... - a pure ORM (no query builder dimension) which allows you to write regular native SQL and receive back properly nested/structured nested pure (not db-aware stateful) business objects.
I am fetching issue where I have to generate dynamic ORDER BY clause for multiple columns.
Here is my sample query:
My Try:
Here, if I pass ORDER BY clause as ${orderBy}, then it take it as a value. It shows error.
The generated sql is like below:
Please help me.