Read from SQL databases

These functions allow you to interact with an SQL database using a Kotlin DataFrame library.

There are two main blocks of available functionality:

Methods for reading data from a database
- readSqlTable reads specific database table
- readSqlQuery executes SQL query
- readResultSet reads from created earlier ResultSet
- readAllSqlTables reads all tables (all non-system tables)
Methods for reading table schemas
- getSchemaForSqlTable for specific tables
- getSchemaForSqlQuery for result of executing SQL queries
- getSchemaForResultSet for created earlier ResultSet
- getSchemaForAllSqlTables for all non-system tables

All methods above can be accessed like DataFrame.getSchemaFor...() via a companion for DataFrame.

Also, there are a few extension functions available on Connection, ResultSet, and DbConnectionConfig objects.

Methods for reading data from a database
- readDataFrame on Connection or DbConnectionConfig converts the result of an SQL query or SQL table to a DataFrame object.
- readDataFrame on ResultSet reads from created earlier ResultSet
Methods for reading table schemas from a database
- getDataFrameSchema on Connection or DbConnectionConfig for an SQL query result or the SQL table
- getDataFrameSchema on ResultSet for created earlier ResultSet

NOTE: This is an experimental module, and for now, we only support four databases: MS SQL, MariaDB, MySQL, PostgreSQL, and SQLite.

Moreover, since release 0.15 we support the possibility to register custom SQL database, read more in our guide.

Additionally, support for JSON and date-time types is limited. Please take this into consideration when using these functions.

Getting started with reading from SQL database in Gradle Project

In the first, you need to add a dependency

implementation("org.jetbrains.kotlinx:dataframe-jdbc:$dataframe_version")

after that, you need to add a dependency for a JDBC driver for the used database, for example

For MariaDB:

implementation("org.mariadb.jdbc:mariadb-java-client:$version")

Maven Central version could be found here.

For PostgreSQL:

implementation("org.postgresql:postgresql:$version")

Maven Central version could be found here.

For MySQL:

implementation("com.mysql:mysql-connector-j:$version")

Maven Central version could be found here.

For SQLite:

implementation("org.xerial:sqlite-jdbc:$version")

Maven Central version could be found here.

For MS SQL:

implementation("com.microsoft.sqlserver:mssql-jdbc:$version")

Maven Central version could be found here.

In the second, be sure that you can establish a connection to the database.

For this, usually, you need to have three things: a URL to a database, a username, and a password.

Call one of the following functions to collect data from a database and transform it to the dataframe.

For example, if you have a local PostgreSQL database named as testDatabase with table Customer, you could read first 100 rows and print the data just copying the code below:

import org.jetbrains.kotlinx.dataframe.io.DbConnectionConfig
import org.jetbrains.kotlinx.dataframe.api.print

val url = "jdbc:postgresql://localhost:5432/testDatabase"
val username = "postgres"
val password = "password"

val dbConfig = DbConnectionConfig(url, username, password)

val tableName = "Customer"

val df = DataFrame.readSqlTable(dbConfig, tableName, 100)

df.print()

Find a full example project here.

Getting Started with Notebooks

To use the latest version of the Kotlin DataFrame library and a specific version of the JDBC driver for your database (MariaDB is used as an example below) in your Notebook, run the following two cells.

First, specify the version of the JDBC driver

USE {
    dependencies("org.mariadb.jdbc:mariadb-java-client:$version")
}

Next, import Kotlin DataFrame library in the cell below.

%use dataframe

NOTE: The order of cell execution is important, the dataframe library is waiting for a JDBC driver to force classloading.

Find a full example Notebook here.

Nullability Inference

Each method has an important parameter called inferNullability.

By default, this parameter is set to true, indicating that the method should inherit the NOT NULL constraints from the SQL table definition.

However, if you prefer to ignore the SQL constraints and determine nullability solely based on the presence of null values in the data, set this parameter to false.

In this case, the column will be considered nullable if there is at least one null value in the data; otherwise, it will be considered non-nullable for the newly created DataFrame object.

Reading Specific Tables

These functions read all data from a specific table in the database. Variants with a limit parameter restrict how many rows will be read from the table.

readSqlTable(dbConfig: DbConnectionConfig, tableName: String, limit: Int, inferNullability: Boolean, dbType: DbType?): AnyFrame

Read all data from a specific table in the SQL database and transform it into an AnyFrame object.

The dbConfig: DbConnectionConfig parameter represents the configuration for a database connection, created under the hood and managed by the library. Typically, it requires a URL, username, and password.

The dbType parameter is the type of database, could be a custom object, provided by user, optional, default is null, to know more, read the guide.

import org.jetbrains.kotlinx.dataframe.io.DbConnectionConfig

val dbConfig = DbConnectionConfig("URL_TO_CONNECT_DATABASE", "USERNAME", "PASSWORD")

val users = DataFrame.readSqlTable(dbConfig, "Users")

The limit: Int parameter allows setting the maximum number of records to be read.

val users = DataFrame.readSqlTable(dbConfig, "Users", limit = 100)

readSqlTable(connection: Connection, tableName: String, limit: Int, inferNullability: Boolean, dbType: DbType?): AnyFrame

Another variant, where instead of dbConfig: DbConnectionConfig we use a JDBC connection: Connection object.

import java.sql.Connection
import java.sql.DriverManager

val connection = DriverManager.getConnection("URL_TO_CONNECT_DATABASE")

val users = DataFrame.readSqlTable(connection, "Users")

connection.close()

Extension functions for reading SQL table

The same example, rewritten with the extension function:

import java.sql.Connection
import java.sql.DriverManager

val connection = DriverManager.getConnection("URL_TO_CONNECT_DATABASE")

val users = connection.readDataFrame("Users", 100)

connection.close()

Connection.readDataFrame(sqlQueryOrTableName: String, limit: Int, inferNullability: Boolean, dbType: DbType?): AnyFrame

Read all data from a specific table in the SQL database and transform it into an AnyFrame object.

sqlQueryOrTableName:String is the SQL query to execute or name of the SQL table.

NOTE: It should be a name of one of the existing SQL tables, or the SQL query should start from SELECT and contain one query for reading data without any manipulation. It should not contain ; symbol.

All other parameters are described above.

DbConnectionConfig.readDataFrame(sqlQueryOrTableName: String, limit: Int, inferNullability: Boolean, dbType: DbType?): AnyFrame

If you do not have a connection object or need to run a quick, isolated experiment reading data from an SQL database, you can delegate the creation of the connection to DbConnectionConfig.

Executing SQL Queries

These functions execute an SQL query on the database and convert the result into a DataFrame object. If a limit is provided, only that many rows will be returned from the result.

readSqlQuery(dbConfig: DbConnectionConfig, sqlQuery: String, limit: Int, inferNullability: Boolean, dbType: DbType?): AnyFrame

Execute a specific SQL query on the SQL database and retrieve the resulting data as an AnyFrame.

import org.jetbrains.kotlinx.dataframe.io.DbConnectionConfig

val dbConfig = DbConnectionConfig("URL_TO_CONNECT_DATABASE", "USERNAME", "PASSWORD")

val df = DataFrame.readSqlQuery(dbConfig, "SELECT * FROM Users WHERE age > 35")

readSqlQuery(connection: Connection, sqlQuery: String, limit: Int, inferNullability: Boolean, dbType: DbType?): AnyFrame

Another variant, where instead of dbConfig: DbConnectionConfig we use a JDBC connection: Connection object.

import java.sql.Connection
import java.sql.DriverManager

val connection = DriverManager.getConnection("URL_TO_CONNECT_DATABASE")

val df = DataFrame.readSqlQuery(connection, "SELECT * FROM Users WHERE age > 35")

connection.close()

Extension functions for reading a result of an SQL query

The same example, rewritten with the extension function:

import java.sql.Connection
import java.sql.DriverManager

val connection = DriverManager.getConnection("URL_TO_CONNECT_DATABASE")

val df = connection.readDataFrame(dbConfig, "SELECT * FROM Users WHERE age > 35", 10)

connection.close()

Reading from ResultSet

These functions read data from a ResultSet object and convert it into a DataFrame. The versions with a limit parameter will only read up to the specified number of rows.

readResultSet(resultSet: ResultSet, dbType: DbType, limit: Int, inferNullability: Boolean): AnyFrame

This function allows reading a ResultSet object from your SQL database and transforms it into an AnyFrame object.

A ResultSet object maintains a cursor pointing to its current row of data. By default, a ResultSet object is not updatable and has a cursor that moves forward only. Therefore, you can iterate it only once and only from the first row to the last row.

More details about ResultSet can be found in the official Java documentation.

Note that reading from the ResultSet could potentially change its state.

The dbType: DbType parameter specifies the type of our database (e.g., PostgreSQL, MySQL, etc.), supported by a library. Currently, the following classes are available: H2, MsSql, MariaDb, MySql, PostgreSql, Sqlite.

Also, users have an ability to pass objects, describing their custom databases, more information in guide.

import org.jetbrains.kotlinx.dataframe.io.db.PostgreSql
import java.sql.ResultSet

val df = DataFrame.readResultSet(resultSet, PostgreSql)

readResultSet(resultSet: ResultSet, connection: Connection, limit: Int, inferNullability: Boolean, dbType: DbType?): AnyFrame

Another variant, we use a JDBC connection: Connection object.

import java.sql.Connection
import java.sql.DriverManager
import java.sql.ResultSet

val connection = DriverManager.getConnection("URL_TO_CONNECT_DATABASE")

val df = DataFrame.readResultSet(resultSet, connection)

connection.close()

Extension functions for reading a result of the SQL query

The same example, rewritten with the extension function:

import java.sql.Connection
import java.sql.DriverManager
import java.sql.ResultSet

val connection = DriverManager.getConnection("URL_TO_CONNECT_DATABASE")

val df = rs.readDataFrame(connection, 10)

connection.close()

ResultSet.readDataFrame(connection: Connection, limit: Int, inferNullability: Boolean, dbType: DbType?): AnyFrame

Reads the data from a ResultSet and converts it into a DataFrame.

connection is the connection to the database (it's required to extract the database type) that the ResultSet belongs to.

Reading Entire Tables

These functions read all data from all tables in the connected database. Variants with a limit parameter restrict how many rows will be read from each table.

readAllSqlTables(dbConfig: DbConnectionConfig, limit: Int, inferNullability: Boolean, dbType: DbType?): Map<String, AnyFrame>

Retrieves data from all the non-system tables in the SQL database and returns them as a map of table names to AnyFrame objects.

import org.jetbrains.kotlinx.dataframe.io.DbConnectionConfig

val dbConfig = DbConnectionConfig("URL_TO_CONNECT_DATABASE", "USERNAME", "PASSWORD")

val dataframes = DataFrame.readAllSqlTables(dbConfig)

readAllSqlTables(connection: Connection, limit: Int, inferNullability: Boolean, dbType: DbType?): Map<String, AnyFrame>

Another variant, where instead of dbConfig: DbConnectionConfig we use a JDBC connection: Connection object.

import java.sql.Connection
import java.sql.DriverManager

val connection = DriverManager.getConnection("URL_TO_CONNECT_DATABASE")

val dataframes = DataFrame.readAllSqlTables(connection)

connection.close()

Schema reading for a specific SQL table

The purpose of these functions is to facilitate the retrieval of table schema. By providing a table name and either a database configuration or connection, these functions return the DataFrameSchema of the specified table.

getSchemaForSqlTable(dbConfig: DbConnectionConfig, tableName: String, dbType: DbType?): DataFrameSchema

This function captures the schema of a specific table from an SQL database.

import org.jetbrains.kotlinx.dataframe.io.DbConnectionConfig

val dbConfig = DbConnectionConfig("URL_TO_CONNECT_DATABASE", "USERNAME", "PASSWORD")

val schema = DataFrame.getSchemaForSqlTable(dbConfig, "Users")

getSchemaForSqlTable(connection: Connection, tableName: String, dbType: DbType?): DataFrameSchema

Another variant, where instead of dbConfig: DbConnectionConfig we use a JDBC connection: Connection object.

import java.sql.Connection
import java.sql.DriverManager

val connection = DriverManager.getConnection("URL_TO_CONNECT_DATABASE")

val schema = DataFrame.getSchemaForSqlTable(connection, "Users")

connection.close()

Schema reading from an SQL query

These functions return the schema of an SQL query result.

Once you provide a database configuration or connection and an SQL query, they return the DataFrameSchema of the query result.

getSchemaForSqlQuery(dbConfig: DbConnectionConfig, sqlQuery: String, dbType: DbType?): DataFrameSchema

This function executes an SQL query on the database and then retrieves the resulting schema.

import org.jetbrains.kotlinx.dataframe.io.DbConnectionConfig

val dbConfig = DbConnectionConfig("URL_TO_CONNECT_DATABASE", "USERNAME", "PASSWORD")

val schema = DataFrame.getSchemaForSqlQuery(dbConfig, "SELECT * FROM Users WHERE age > 35")

getSchemaForSqlQuery(connection: Connection, sqlQuery: String, dbType: DbType?): DataFrameSchema

Another variant, where instead of dbConfig: DbConnectionConfig we use a JDBC connection: Connection object.

import java.sql.Connection
import java.sql.DriverManager

val connection = DriverManager.getConnection("URL_TO_CONNECT_DATABASE")

val schema = DataFrame.getSchemaForSqlQuery(connection, "SELECT * FROM Users WHERE age > 35")

connection.close()

Extension functions for schema reading from an SQL query or an SQL table

The same example, rewritten with the extension function:

import java.sql.Connection
import java.sql.DriverManager

val connection = DriverManager.getConnection("URL_TO_CONNECT_DATABASE")

val schema = connection.getDataFrameSchema("SELECT * FROM Users WHERE age > 35")

connection.close()

Connection.getDataFrameSchema(sqlQueryOrTableName: String, dbType: DbType?): DataFrameSchema

Retrieves the schema of an SQL query result or an SQL table using the provided database configuration.

DbConnectionConfig.getDataFrameSchema(sqlQueryOrTableName: String, dbType: DbType?): DataFrameSchema

Retrieves the schema of an SQL query result or an SQL table using the provided database configuration.

The dbConfig: DbConnectionConfig represents the configuration for a database connection, created under the hood and managed by the library. Typically, it requires a URL, username, and password.

import org.jetbrains.kotlinx.dataframe.io.DbConnectionConfig

val dbConfig = DbConnectionConfig("URL_TO_CONNECT_DATABASE", "USERNAME", "PASSWORD")

val schema = dbConfig.getDataFrameSchema("SELECT * FROM Users WHERE age > 35")

Schema reading from ResultSet

These functions return the schema from a ResultSet provided by the user.

This can help developers infer the structure of the result set, which is quite essential for data transformation and mapping purposes.

getSchemaForResultSet(resultSet: ResultSet, dbType: DbType): DataFrameSchema

This function reads the schema from a ResultSet object provided by the user.

Also, users have an ability to pass objects, describing their custom databases, more information in guide.

import org.jetbrains.kotlinx.dataframe.io.db.PostgreSql
import java.sql.ResultSet

val schema = DataFrame.getSchemaForResultSet(resultSet, PostgreSql)

Extension functions for schema reading from the ResultSet

The same example, rewritten with the extension function:

import org.jetbrains.kotlinx.dataframe.io.db.PostgreSql
import java.sql.ResultSet

val schema = resultSet.getDataFrameSchema(PostgreSql)

based on

ResultSet.getDataFrameSchema(dbType: DbType): DataFrameSchema

Schema reading for all non-system tables

These functions return a list of all DataFrameSchema from all the non-system tables in the SQL database. They can be called with either a database configuration or a connection.

getSchemaForAllSqlTables(dbConfig: DbConnectionConfig, dbType: DbType?): Map<String, DataFrameSchema>

This function retrieves the schema of all tables from an SQL database and returns them as a map of table names to DataFrameSchema objects.

import org.jetbrains.kotlinx.dataframe.io.DbConnectionConfig

val dbConfig = DbConnectionConfig("URL_TO_CONNECT_DATABASE", "USERNAME", "PASSWORD")

val schemas = DataFrame.getSchemaForAllSqlTables(dbConfig)

getSchemaForAllSqlTables(connection: Connection, dbType: DbType?): Map<String, DataFrameSchema>

This function retrieves the schema of all tables using a JDBC connection: Connection object and returns them as a list of DataFrameSchema.

import java.sql.Connection
import java.sql.DriverManager

val connection = DriverManager.getConnection("URL_TO_CONNECT_DATABASE")

val schemas = DataFrame.getSchemaForAllSqlTables(connection)

connection.close()

Last modified: 14 January 2025