Order of tables is important if (any of the following):
•LEFT join is used, or
•CONCAT aggregation is used for any column, or
•COALESCE aggregation is used
LEFT join is sensitive to the order, because the most left-table will be used as a join master, and switching the tables may produce different results in the output table.
CONCAT and COALESCE are also sensitive to the order because they process the value from left to right.
The joining order is determined by the order they appear in JSON file. You do not need to do anything extra if that order is the same for all conflicting columns in the joined tables. However, there may be a need to define a custom joining order on a column basis, which is described in the next chapter.
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Note: The tables matched by wildcard joining (for example Table*) are ordered in an non-deterministic way if more than one table matches the wildcard. |
