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Pierre Cornelius. Join our community to see this answer! A RIGHT OUTER JOIN causes the query to output a set consisting of fully populated columns where matching rows were found as in the inner join and also partly populated rows for each instance where a right-side row exists with no corresponding set in the left-side stream. It returns one row for each pair of matching streams and one partly populated row for each row in each stream where matches are not found. It combines the behaviors of the right and left joins.
That is, for each row in the left stream, an output stream will be generated for each row in the right stream. In Firebird, you always need to cite the join conditions. Various database theory texts will tell you that ambiguity can exist only where the same column names occur in multiple streams. The practitioner may have a different story to tell. The question of sorting out ambiguities gets down to the way database engines implement the parsing, streaming, and sorting that goes on during a join operation.
InterBase was lenient about the use of disambiguating syntax in joins, with sometimes unfortunate results. If you are migrating your existing application code from an InterBase heritage, do not feel too bad about the SQL exceptions that Firebird throws up during the first test run through queries with joins.
It is showing you places in your code where, in the past, you have been allowed to submit queries that could produce wrong output.
Firebird will not accept JOIN queries containing column references that do not have full, consistent table qualification. Table qualification can be by table identifier or by table aliasing. From version 1. If you are using version 1. The preceding examples used the table identifier. When the name of the table is long or complicated, or there are many tables, table aliases are useful and, in some cases, essential to clarify statements.
The query engine treats a table alias as a synonym of the table it represents. Table aliases are mandatory in queries that form multiple streams from within the same table. Mixing table identifiers with aliases in the same query will cause exceptions from Firebird 1.
The following example uses table identifiers:. Use any useful string composed of up to 31 of the characters that are valid for metadata qualifiers i. Spaces, diacritics, double-quoted alias names, and alias names that begin with a numeral are not legal. When reading through a stream, the database engine implements a pointer, whose address changes as the read advances from top to bottom.
Internal cursors are not accessible to clients except through the medium of join and subquery syntax. The current address of an internal cursor is an absolute offset from the address of the first stream in the set, which means it can only advance forward. Internally, the optimizer utilizes indexes and organizes input streams into a plan , to ensure that a query begins returning output in the shortest possible time.
In any multi-table operation, the Firebird engine maintains one internal cursor for each stream. When a match occurs, the engine creates a merged image for the output stream by copying the streams from the current addresses of the two cursors, as shown in Figure. Design conditions sometimes require forming a joined set from two or more streams that come from the same table.
The popular term for this is self-referencing join. The term re-entrant join is morphologically more appropriate, since there are other types of re-entrant query that do not involve joins. To perform the re-entrant join, the engine maintains one internal cursor for each stream, within the same table image. Conceptually, it treats the two cursor contexts as though they were separate tables.
In this situation, the syntax of the table references uses mandatory aliasing to distinguish the cursors for the two streams. In the following example, each department in an organization is stored with a parent identifier that points to the primary key of its supervising department. The query treats the departments and the parent departments as if they were two tables:. The output from this query is very simple: a one-level denormalization. Tree output operations on tables with this kind of structure are often recursive, using stored procedures to implement and control the recursions.
Firebird Interview Questions. ID where A. Mark Rotteveel Azad Azad 4, 3 3 gold badges 27 27 silver badges 53 53 bronze badges. Please only add relevant tags to your question — Mark Rotteveel. Add a comment. Active Oldest Votes. Mark Rotteveel Mark Rotteveel Azad 4, 3 3 gold badges 27 27 silver badges 53 53 bronze badges. Robles Robles 31 5 5 bronze badges. Ersin Tarhan Ersin Tarhan 2 2 silver badges 11 11 bronze badges.
ErsinTarhan The question is about Firebird where this syntax doesn't work, the question was originally incorrectly tagged with sql-server as well.
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