You can use the Match Keys property to select pairs of input fields to match as keys for joining pairs of data sets. The Match Keys property is found on a number of nodes, for example the Merge and Join nodes.
If you have connected the node to an output which contains a data set, you can select input fields from the list. If there is no input data, then you can type in the name of fields. It is strongly recommended to connect the node to provide input data, and then choose fields from the list, to avoid mistakes such as misspelling field names.
If you manually type in names, once you connect an output pin with data, the fields that you entered will be checked against those which are available. Fields which do not exist on the input data set will be shown in red and should be removed.
Each pair of fields must be of a similar type, otherwise they will be highlighted in red and the node will generate an error. You can compare the number types int, long and double against each other, or string and Unicode against each other.
For each pair of fields, you can optionally choose whether to reverse the sort order for comparisons, and for string fields you can choose whether to compare case-insensitively or by substring. If you set any of these options, then the context menu button will be shown on a blue background. This menu also allows you to remove fields from the property.
Choosing the Compare Substrings option will prompt you to specify the start position, and an optional end position, for each of the string fields.
The Advanced tab allows you to enter a Python expression to identify the fields to use for the Left Key and Right Key. If you have already chosen fields using the Basic view then you will be able to see the script equivalent for your choices when you switch to the Advanced view.