You can specify a single table or view, multiple tables, or a single view and multiple tables. When you specify more than one table, the tables are said to be joined.

The following example illustrates a two-table outer join:

The following example shows an outer join embedded in a vendor string. The letters “OJ” can be either upper or lower case.

PSQL supports two-table outer joins as specified in the Microsoft ODBC documentation. In addition to simple two-table outer joins, PSQL supports n-way nested outer joins.

The outer join may or may not be embedded in a vendor string. If a vendor string is used, PSQL strips it off and parses the actual outer join text.

The PSQL Database Engine has implemented LEFT OUTER JOIN using SQL92 (SQL2) as a model. The syntax is a subset of the entire SQL92 syntax which includes cross joins, right outer joins, full outer joins, and inner joins. The TableRefList below occurs after the FROM keyword in a SELECT statement and before any subsequent WHERE, HAVING, and other clauses. Note the recursive nature of TableRef and LeftOuterJoin – a TableRef can be a left outer join that can include TableRefs which, in turn, can be left outer joins and so forth.

The search condition (SearchCond) contains join conditions which in their usual form are LT.ColumnName = RT.ColumnName, where LT is left table, RT is right table, and ColumnName represents some column within a given domain. Each predicate in the search condition must contain some nonliteral expression.

The implementation of left outer join goes beyond the syntax in the Microsoft ODBC documentation.

The syntax in the previous section includes but goes beyond the ODBC syntax in the Microsoft ODBC documenation. Furthermore, the vendor string escape sequence at the beginning and end of the left outer join does not change the core syntax of the outer join.

The PSQL Database Engine accepts outer join syntax without the vendor strings. However, for applications that want to comply with ODBC across multiple databases, the vendor string construction should be used. Because ODBC vendor string outer joins do not support more than two tables, it may be necessary to use the syntax shown following Table 30.

The following four tables are used in these examples.

The following example shows a simple two-way Left Outer Join:

This two-way outer join produces the following result set:

Notice the NULL entry for Franky Avalon in the table. That is because no DeptID of D103 was found in the Dept table. In a standard (INNER) join, Franky Avalon would have been dropped from the result set altogether.

The algorithm that the PSQL Engine uses for the previous example is as follows:
taking the left table, traverse the right table, and for every case where the ON condition is TRUE for the current right table row, return a result set row composed of the appropriate right table row appended to the current left-table row.

If there is no right table row where the ON condition is TRUE, (it is FALSE for all right table rows given the current left table row), create a row instance of the right table with all column values NULL.

That result set, combined with the current left-table row for each row, is indexed in the returned result set. The algorithm is repeated for every left table row to build the complete result set. In the simple two-way left outer join shown previously, Emp is the left table and Dept is the right table.

With radiating left outer joins, all other tables are joined onto one central table. In the following example of a three-way radiating left outer join, Emp is the central table and all joins radiate from that table.

In a chaining left outer join, one table is joined to another, and that table, in turn, is joined to another. The following example illustrates a three-way chaining left outer join:

This join could also be expressed as:

We recommend the first syntax because it lends itself to both the radiating and chaining joins. This second syntax cannot be used for radiating joins because nested left outer join ON conditions cannot reference columns in tables outside their nesting. In other words, in the following query, the reference to Emp.EmpID is illegal:

The following example shows a three-way radiating left outer join, less optimized:

This query returns the same results as shown in Table 33, assuming there are no NULL values for EmpID in Emp and EmpID is a unique valued column. This query, however, is not optimized as well as the one show for Table 33 and can be much slower.

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