Автор: which displays per-product sales totals in only the top sales regions. Which have clause defines two auxiliary statements named regional_transformation and top_regions, where the output of regional_sales is used in top_regions and the output of top_regions is used in the priple could have been written without That have, but we’d have needed two levels of nested sub-Discovers.
Although not, often a period does not involve productivity rows that are totally duplicate: it may be needed seriously to see a single or several industries to find out if a similar point might have been achieved prior to
optional RECURSIVE modifier changes That have from a mere syntactic convenience into a feature that accomplishes things not otherwise possible in standard SQL. Using RECURSIVE, a That have query can refer to its own output. A very simple example is this query to sum the integers from 1 through 100:
general form of a recursive Which have query is always a non-recursive term, then Connection (or Commitment Most of the), then a recursive term, where only the recursive term can contain a reference to the query’s own output. Such a query is executed as follows:
Evaluate the non-recursive term. For Commitment (but not Connection Every), discard duplicate rows. Include all remaining rows in the result of the recursive query, and also place them in a temporary working table.
Evaluate the recursive term, substituting the current contents of the working table for the recursive self-reference. For Connection (but not Connection All the), discard duplicate rows and rows that duplicate any previous result row. Include all remaining rows in the result of the recursive query, and also place them in a temporary intermediate table.
Note: Strictly speaking, this process is iteration not recursion, but RECURSIVE is the terminology chosen by the SQL standards committee.
In the example above, the working table has just a single row in each step, and it takes on the values from 1 through 100 in successive steps. In the 100th step, there is no output because of the Where clause, and so the query terminates.
Recursive question are generally always handle hierarchical otherwise forest-structured data. A helpful example is this inquire to get the head and you may secondary sub-parts of something, considering just a table that shows quick inclusions:
When working with recursive queries it is important to be sure that the recursive part of the query will eventually return no tuples, or else the query will loop indefinitely. Sometimes, using Partnership instead of Partnership The can accomplish this by discarding rows that duplicate previous output rows. standard method for handling such situations is to compute an array of the already-visited values. For example, consider the following query that searches a table graph using a hook up field:
This query will loop if the link relationships contain cycles. Because we require a «depth» output, just changing Connection All of the to Commitment would not eliminate the looping. Instead we need to recognize whether we have reached the same row again while following a particular roadway of links. We add two columns path and cycle to the loop-prone query:
Except that stopping schedules, the new assortment well worth often is helpful in its correct because symbolizing new «path» brought to reach any form of row.
In the general case where more than one field needs to be checked to recognize a cycle, use an array of rows. For example, if we needed to compare fields f1 and f2:
The
Tip: Omit the ROW() syntax in the common case where only one field needs to be checked to recognize a cycle. This allows a simple array rather than a composite-type array to be used, gaining efficiency.
