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Solidity to SQL types

Review the best practices for Solidity types and SQL compatibility.

To prevent overflows while working with Solidity numbers, it is recommended to use a text type in certain scenarios. Anything larger than a uint64 / int32 could lead to an overflow in the SQLite database. Note that in many use cases, it is unlikely overflows will happen due to the extremely large size of these numbers.

Alternatively, consider casting the overflow-able numbers to or simply use a int64 in smart contracts if it makes sense for the use case.

Type conversion

See the following tables for how each Solidity number should be defined in Tableland schemas:

Solidity TypeSQL Type
Solidity TypeSQL Type

Other best practices have also been defined below:

Solidity TypeSQL Type

Tableland doesn’t support boolean values, but TRUE/FALSE keywords are supported since they're aliased to a 1 or 0. Thus, a Solidity bool should be defined as an integer in Tableland such that there is no need to wrap the Solidity SQL statement's TRUE/FALSE in single quotes (e.g., INSERT INTO ... VALUES (TRUE, FALSE);).

Strings & casting

A common practice is to use the Strings library by OpenZeppelin when writing SQL statements in Solidity. This library has a number of useful methods, including toString, which takes a uint256 and converts it into the string type.

For example, a quick and dirty "simple" cast from a uint to a uint256 before calling the Strings.toString() method could resemble the following. The multipleToString method is simply an example of how to concatenate multiple strings together, which is often useful when generating SQL statements that use variables. Basically, use the string casting and abi.encodePacked with a comma-separated list of values with type string.

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.4;

import "@openzeppelin/contracts/utils/Strings.sol";

contract SimpleCast {
function uint8ToString(uint8 _i) internal pure returns (string memory str) {
return string(Strings.toString(_i));
function multipleToString(uint8 _i, uint8 _j) internal pure returns (string memory str) {
return string(abi.encodePacked(Strings.toString(_i), " and ", Strings.toString(_j)));

It's possible to cast any unsigned integer to a uint256 using uint256(<number_here>). Since the toString method takes uint256, the casting is performed implicitly, meaning, uint256(_i) is unneeded in the example above (i.e., no need for string(Strings.toString(uint256(_i)))).

For math reasons, there exists a SafeCast library as well, which handles a full suite of number conversion possibilities. There are safety considerations to take into account since Solidity does not revert on overflows, so it’s always recommended to proceed with caution and ensure the proper testing has been performed in casting scenarios. Below is an example of how to use SafeCast to convert a signed integer int8 to an unsigned uint256, thus, enabling int8 to be used by the Strings library. Note that it still cannot convert a negative number into a positive one without some additional logic:

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.4;

import "@openzeppelin/contracts/utils/Strings.sol";
import "@openzeppelin/contracts/utils/math/SafeCast.sol";

contract SimpleCast {
function positiveInt8ToString(int8 _i) internal pure returns (string memory str) {
return string(Strings.toString(SafeCast.toUint256(_i)));
function anyInt8ToString(int8 _i) internal pure returns (string memory str) {
if(_i < 0) {
return string(abi.encodePacked('-', Strings.toString(SafeCast.toUint256(_i * -1))));
return string(Strings.toString(SafeCast.toUint256(_i)));

For reference, the SafeCast library comes equipped with all potential casting possibilities needed for number conversion. This includes the following:

  • toUint8
  • toUint16
  • toUint32
  • toUint64
  • toUint128
  • toUint256
  • toInt8
  • toInt16
  • toInt32
  • toInt64
  • toInt128
  • toInt256

More information can be found in the SafeCast documentation: here.

Addresses & casting

The Strings library also comes with a toHexString method. This can be used to easily convert an address to a string and insert the value into a table. As a best practice, the address should be of type text in the Tableland world. For example, take some method that allows the calling address _addr to insert itself into a table—assume the table's schema is a simple address text:

function insertAddress(
address _addr,
unint256 tableId,
string tableName,
address tableland
) public {
string memory addr = Strings.toHexString(_addr);
// INSERT INTO {tableName} VALUES ('{addr}');