"SQL" (External Evaluation System)

Listing of Supported Databases »


ExternalEvaluate Usage

  • ExternalEvaluate ["SQL", code ] executes a string of SQL in a database connection and returns the result as a Wolfram Language expression.
  • ExternalEvaluate ["SQL" form , code ] executes the SQL string and returns the result in the specified form . Possible specifications for form are "Dataset", "Rows", "NamedRows", "Columns" and "NamedColumns".
  • ExternalEvaluate [ DatabaseReference [ ref ] , code ] is equivalent to ExternalEvaluate [ {"SQL","Target" DatabaseReference [ ref ] } , code ] .
  • The possible settings for target in ExternalEvaluate [ {"SQL","Target" target } , ] include:
  • "path"or File ["path"] path to an SQLite database
    URL ["url"] a connection specified in the form "backend://user:password@host:port/name"
    DatabaseReference [ ] an SQL database connection
    SQLConnection [ ] an SQL-JDBC database connection

Data Types

Usage Notes

  • String templates (<**>) can be used to evaluate and insert Wolfram Language expressions into the SQL code string.


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Basic Examples    (2)

Specify a reference to the demo database:

Evaluate a query in SQL and return the result:

To use SQL in an external language cell, you need to register a default database:

Type > and select SQL from the drop-down menu to get an SQL code cell:

SELECT * FROM employees LIMIT 10

Scope    (4)

Start a session:

Evaluate a query that is returning all tables:

Query a certain table using limit:

Rename columns using an AS statement:

Close the session:

By default ExternalEvaluate returns data using Dataset :

"ReturnType" can be used to return data in a different form:

While using "Columns", numerical data is returned as a packed array when possible:

Start a session to a local PostgreSQL database (to evaluate this input, you would need to have an appropriate PostgreSQL database instance running):

When supported, ExternalEvaluate will return Wolfram Language expressions instead of strings:

String templates can be used to insert Wolfram Language expressions into SQL code:

Set two variables:

The expression x^2+y^2 is evaluated in the Wolfram Language, and the result is converted and inserted into the SQL code string:

Manually provide arguments to the template by using an Association :

Use named arguments:

Parameters are automatically normalized according to the database backend that is currently in use. Expressions like Integer , ByteArray , String , DateObject and TimeObject can be used if the backend supports them:

Applications    (2)

DatabaseReference can also represent an in-memory SQLite database:

All operations done during an in-memory session are lost at the end of an ExternalEvaluate call:

Operations that are doing side effects on the database typically return Null :

For some backends, it is possible to insert data and specify a return value:

Possible Issues    (1)

Usually a database returns dates using the server time zone:

Change the query to return a column at a particular time zone:

Or "SessionEpilog" can be used to set a default time zone: