Skip to content
Switch branches/tags

Name already in use

A tag already exists with the provided branch name. Many Git commands accept both tag and branch names, so creating this branch may cause unexpected behavior. Are you sure you want to create this branch?
Go to file
Cannot retrieve contributors at this time


Working with Eclipse

Developing SQLancer using Eclipse is expected to work well. You can import SQLancer with a single step:

File -> Import -> Existing Maven Projects -> Select the SQLancer directory as root directory -> Finish

If you do not find an option to import Maven projects, you might need to install the M2Eclipse plugin.

Implementing Support for a New DBMS

The DuckDB implementation provides a good template for a new implementation. The DuckDBProvider class is the central class that manages the creation of the databases and executes the selected test oracles. Try to copy its structure for the new DBMS that you want to implement, and start by generate databases (without implementing a test oracle). As part of this, you will also need to implement the equivalent of DuckDBSchema, which represents the database schema of the generated database. After you can successfully generate databases, the next step is to generate one of the test oracles. For example, you might want to implement NoREC (see DuckDBNoRECOracle or DuckDBQueryPartitioningWhereTester for TLP). As part of this, you must also implement a random expression generator (see DuckDBExpressionGenerator) and a visitor to derive the textual representation of an expression (see DuckDBToStringVisitor).

Typed vs. Untyped Expression Generation

Each DBMS implementation provides an expression generator used, for example, to generate expressions used in WHERE clauses. We found that DBMS can be roughly classified into "permissive" ones, which apply implicit type conversions when needed and "strict" ones, which provide only few implicit conversions and output an error when the type is unexpected. For example, consider the following test case:


If the test case is executed using MySQL, which is a permissive DBMS, the SELECT fetches a single row, since the content of the c0 value is interpreted as a boolean. If the test case is executed using PostgreSQL, which is a strict DBMS, the SELECT is not accepted as a valid query, and PostgreSQL outputs an error "argument of WHERE must be type boolean". The implementation of the expression generator depends on whether we are dealing with a permissive or a strict DBMS. Since SQLancer's main goal is to find logic bugs, we want to generate as many valid queries as possible.

For a permissive DBMS, implementing the expression generator is easier, since the expression generator does not need to care about the type of the expression, since the DBMS will apply any necessary conversions implicitly. For MySQL, the main generateExpression method thus does not accept any type as an argument (see MySQLExpressionGenerator). This method can be called when a expression is required for, for example, a WHERE clause. In principle, this approach can also be used for strict DBMS, by adding errors such as argument of WHERE must be type boolean to the list of expected errors. However, using such an "untyped" expression generator for a strict DBMS will result in many semantically invalid queries being generated.

For a strict DBMS, the better approach is typically to attempt to generate expressions of the expected type. For PostgreSQL, the expression generator thus expects an additional type argument (see PostgreSQLExpressionGenerator). This type is propagated recursively. For example, if we require a predicate for the WHERE clause, we pass boolean as a type. The expression generator then calls a method generateBooleanExpression that attempts to produce a boolean expression, by, for example, generating a comparison (e.g., <=). For the comparison's operands, a random type is then selected and propagated. For example, if an integer type is selected, then generateExpression is called with this type once for the left operand, and once for the right operand. Note that this process does not guarantee that the expression will indeed have the expected type. It might happen, for example, that the expression generator attempts to produce an integer value, but that it produces a double value instead, namely when an integer overflow occurs, which, depending on the DBMS, implicitly converts the result to a floating-point value.

Unfixed Bugs

Often, some bugs are fixed only after an extended period, meaning that SQLancer will repeatedly report the same bug. In such cases, it might be possible to avoid generating the problematic pattern, or adding an expected error with the internal error message. Rather than, for example, commenting out the code with the bug-inducing pattern, a pattern implemented by the TiDBBugs class should be applied. The core idea is to use a public, static flag for each issue, which is set to true as long as the issue persists (e.g., see bug35652). The work-around code is then executed—or the problematic pattern should not be generated—if the flag is set to true (e.g., an expected error is added for bug35652). This makes it easy to later on identify and remove all such work-around code once the issue has been fixed.


SQLancer uses JCommander for handling options. The MainOptions class contains options that are expected to be supported by all DBMS-testing implementations. Furthermore, each *Provider class provides a method to return an additional set of supported options.

An option can include lowercase alphanumeric characters, and hyphens. The format of the options is checked by a unit test.

Continuous Integration and Test Suite

To improve and maintain SQLancer's code quality, we use multiple tools:

  • The Eclipse code formatter, to ensure a consistent formatting (Run mvn formatter:format to format all files).
  • Checkstyle, to enforce a consistent coding standard.
  • PMD, which finds programming flaws using static analysis.
  • SpotBugs, which also uses static analysis to find bugs and programming flaws.

You can run them using the following command:

mvn verify

We use GitHub Actions to automatically check PRs.


As part of the GitHub Actions check, we use smoke testing by running SQLancer on each supported DBMS for some minutes, to test that nothing is obviously broken. For DBMS for which all bugs have been fixed, we verify that SQLancer cannot find any further bugs (i.e., the return code is zero).

In addition, we use unit tests to test SQLancer's core functionality, such as random string and number generation as well as option passing. When fixing a bug, add a unit test, if it is easily possible.

You can run the tests using the following command:

mvn test

Note that per default, the smoke testing is performed only for embedded DBMS (e.g., DuckDB and SQLite). To run smoke tests also for the other DBMS, you need to set environment variables. For example, you can run the MySQL smoke testing (and no other tests) using the following command:

MYSQL_AVAILABLE=true mvn -Dtest=TestMySQL test

For up-to-date testing commands, check out the .github/workflows/main.yml file.


Reviewing is an effective way of improving code quality. Everyone is welcome to review any PRs. Currently, all PRs are reviewed at least by the main contributor, @mrigger. Contributions by @mrigger are currently not (necessarily) reviewed, which is not ideal. If you are willing to regularly and timely review PRs, indicate so in the SQLancer Slack workspace.

Naming Conventions

Each class specific to a DBMS is prefixed by the DBMS name. For example, each class specific to SQLite is prefixed by SQLite3. The naming convention is automatically checked.

Commit History

Please pay attention to good commit messages (in particular subject lines). As basic guidelines, we recommend a blog post on How to Write a Git Commit Message written Chris Beams, which provides 7 useful rules. Implement at least the following of those rules:

  1. Capitalize the subject line. For example, write "Refactor the handling of indexes" rather than "refactor the handling of indexes".
  2. Do not end the subject line with a period. For example, write "Refactor the handling of indexes" rather than "Refactor the handling of indexes.".
  3. Use the imperative mood in the subject line. For example, write "Refactor the handling of indexes" rather than "Refactoring" or "Refactored the handling of indexes".

Please also pay attention to a clean commit history. Rather than merging with the main branch, use git rebase to rebase your commits on the main branch. Sometimes, it might happen that you discover an issue only after having already created a commit, for example, when an issue is found by mvn verify in the CI checks. Do not introduce a separate commit for such issues. If the issue was introduced by the last commit, you can fix the issue, and use git commit --amend to change the latest commit. If the change was introduced by one of the previous commits, you can use git rebase -i to change the respective commit. If you already have a number of such commits, you can use git squash to "collapse" multiple commits into one. For more information, you might want to read How (and Why!) to Keep Your Git Commit History Clean written by Kushal Pandya.