Short Description
    DatabaseReader Attributes
    Best Practices
    See also

    Short Description

    DatabaseReader unloads data from database using JDBC driver. Supports Amazon Redshift, Microsoft Access, Microsoft SQL Server, MySQL, Oracle, PostgreSQL, Snowflake, SQLite, Sybase, Vertica, DB2 and any other database with JDBC compliant driver.

    ComponentData sourceInput portsOutput portsEach to all outputsDifferent to different outputsTransformationTransf. req.JavaCTLAuto-propagated metadata


    Port typeNumberRequiredDescriptionMetadata

    Incoming queries to be used in the SQL query attribute. When the input port is connected, Query URL should be specified as e.g. port:$0.fieldName:discrete. See Reading from Input Port.

    Output0for correct data recordsequal metadata
    1-nfor correct data records


    DatabaseReader does not propagate metadata.

    DatabaseReader has no metadata templates.

    Output metadata can use Autofilling Functions

    DatabaseReader Attributes

    AttributeReqDescriptionPossible values
    DB connectionAn ID of a database connection to be used to access the database 
    SQL query[ 1]

    The SQL query defined in the graph. For detailed information, see SQL Query Editor.

    Query URL[ 1]The name of an external file, including the path, defining the SQL query. 
    Query source charset 

    Encoding of an external file defining the SQL query.

    The default encoding depends on DEFAULT_CHARSET_DECODER in defaultProperties.

    UTF-8 | <other encodings>
    Data policy  Determines what should be done when an error occurs. For more information, see Data Policy.

    Strict (default) | Controlled[ 2] | Lenient

    Print statement If enabled, SQL statements will be written to the log.false (default) | true
    Fetch size Specifies the number of records that should be fetched from the database at once.20 | 1-N
    Incremental file[ 3]

    The name of the file storing the incremental key, including the path. See Incremental Reading.

    Incremental key[ 3]

    A variable storing the position of the last read record. See Incremental Reading.

    Auto commit  

    By default, your SQL queries are committed immediately. If you need to perform more operations inside one transaction, switch this attribute to false.

    true (default) | false

    [ 1] At least one of these attributes must be specified. If both are defined, only Query URL is applied.

    [ 2] Controlled data policy in DatabaseReader does not send error records to the edge. Errors are written into the log.

    [ 3] Either both or neither of these attributes must be specified.


    DatabaseReader unloads data from a database table using an SQL query or by specifying a database table and defining a mapping of database columns to Clover fields. It can send unloaded records to all connected output ports.

    Defining Query Attributes

    • Query Statement without Mapping

      When the order of CloverDX metadata fields and database columns in select statement is the same and data types are compatible, implicit mapping can be used which performs positional mapping. A standard SQL query syntax should be used:

      • select * from table [where dbfieldJ = ? and dbfieldK = somevalue]

      • select column3, column1, column2, ... from table [where dbfieldJ = ? and dbfieldK = somevalue]

      For information about how an SQL query can be defined, see SQL Query Editor.

    • Query Statement with Mapping

      If you want to map database fields to Clover fields even for multiple tables, the query will look like this:

      select $cloverfieldA:=table1.dbfieldP, $cloverfieldC:=table1.dbfieldS, ... , $cloverfieldM:=table2.dbfieldU, $cloverfieldM:=table3.dbfieldV from table1, table2, table3 [where table1.dbfieldJ = ? and table2.dbfieldU = somevalue]

      For information about how an SQL query can be defined, see SQL Query Editor.

    Dollar Sign in DB Table Name

    • A single dollar sign in a table name must be escaped by another dollar sign; therefore every dollar sign in a database table name will be transformed to a double dollar sign in the generated query. Meaning that each query must contain an even number of dollar signs in the DB table (consisting of adjacent pairs of dollars).

    SQL Query Editor

    For defining the SQL query attribute, SQL query editor can be used.

    The editor opens after clicking the SQL query attribute row:

    On the left side, there is the Database schema pane containing information about schemas, tables, columns, and data types of these columns.

    Displayed schemas, tables, and columns can be filtered using the values in the ALL combo, Filter in view text area, Filter and Reset buttons, etc.

    You can select any columns by expanding schemas, tables and clicking Ctrl+Click on desired columns.

    Adjacent columns can also be selected by clicking Shift+Click on the first and the last item.

    Then you need to click Generate after which a query will appear in the Query pane.

    Generated Query with Question Marks

    Figure 55.3. Generated Query with Question Marks

    A query may contain question marks if any DB columns differ from output metadata fields. Output metadata are visible in the Output metadata pane on the right side.

    Drag and drop the fields from the Output metadata pane to the corresponding places in the Query pane and then manually remove the "$?:=" characters. See the following figure:

    Generated Query with Output Fields

    Figure 55.4. Generated Query with Output Fields

    You can also type a where statement to the query.

    The buttons underneath allow you to validate the query (Validate) or view data in the table (View).


    Read Records from Database
    Read Query from Input Port

    Read Records from Database

    By generating a query in DatabaseReader, read the name, location and height in feet of mountains from the MountainsDB database.


    Use the DB connection and SQL query attributes.

    DB connectionSee Creating Internal Database Connections.
    SQL queryUse the Generate query button in the SQL query editor.
    Reading records from database

    Figure 55.5. Reading records from database

    In the output metadata, create the name, location and heightft fields. Set their data types to string, string and integer respectively.

    Click on the SQL query property and open the SQL query editor. Select the MountainDB database in the Database schema pane.

    Select the mountain, location and heightft fields and click the Generate query button.


    If the output metadata fields have different names and/or data types, the generated query will contain question mark(s). See SQL Query Editor.


    You can modify the generated query by adding other keywords, e.g. ASC, DESC, etc.

    You can validate the generated query and view the results by clicking the respective buttons in the lower left side of the SQL query editor.

    Set the File URL path of the FlatFileWriter to the external file.

    Read Query from Input Port

    A query is automatically generated into an external file. Read the query from the file and write the results into another file.


    Use the DB connection and Query URL attributes.

    DB connectionSee Creating Internal Database Connections.
    Query URLport:$0.field1:discrete
    Reading query from input port

    Figure 55.6. Reading query from input port

    Set the File URL path of the FlatFileReader to the external file containing the query.

    According to the table above, set the DB connection and Query URL attributes of the DatabaseReader.

    Set the File URL path of the FlatFileWriter to an external file of your choice.

    Input metadata should contain one field in which the query will be written.

    Output metadata should contain a number of fields equivalent to columns selected in the query.


    Make sure that the EOF as delimiter property in the input metadata is set to true.


    DatabaseReader can only read one query per source file.

    Incremental reading

    Incremental Reading allows you to read only new records from a database. This can be done by setting the Incremental key and Incremental file attributes, and editing the Query attribute.

    Let us have a database of customers. Each row in the database consists of an id, date, first name and last name, for example:

    1|2018-02-01 23:58:02|Rocky|Whitson
    2|2018-02-01 23:59:56|Marisa|Callaghan
    3|2018-03-01 00:03:12|Yaeko|Gonzale
    4|2018-03-01 00:15:41|Jeana|Rabine
    5|2018-03-01 00:32:22|Daniele|Hagey

    Read the records, then add a new record to the database and run the graph again reading only the new record.


    In the output metadata, create the id, date, firstName and lastName fields. Set their data types to integer, date, string and string, respectively.

    Use the Incremental key and Incremental file attributes.

    Incremental keykey01="LAST(id)"[a]
    Incremental file${DATATMP_DIR}/customers_inc_key

    [a] Follow the instructions in Incremental Reading to create the Incremental key and edit the Query attribute.

    After the first read, the output file contains five records.

    Incremental reading - first read

    Figure 55.7. Incremental reading - first read

    Now, add a new record to the database, for example:

    6|2018-03-01 00:51:31|Nathalie|Mangram

    and run the graph again.

    This time, only the new record is written to the output file, ignoring the previously processed records.

    Incremental reading - second read

    Figure 55.8. Incremental reading - second read

    Best Practices

    If the Query URL attribute is used, we recommend to explicitly specify Query source charset.


    VersionCompatibility Notice
    5.3.0DBInputTable was renamed to DatabaseReader.