The languages table is just a list of possible language names and a unique language id :. When defining a side to your OUTER JOIN , you are specifying which table will always return its row even if the opposing table on the other side of the join has missing or null values as part of the joining condition.
Therefore, in this case, because we want to ensure that our languages table is the optional table that can return null values during this comparison, we swapped the order of the tables in this conditional, so languages is on the left and is optional while books is on the right.
This means that this snippet of the query:. Getting a crosstab format table into a tabular format can be done with many queries and UNIONs or Chartio has a Data Pipeline step that can help you accomplish this task. We will create two new tables with the same structure for the demonstration:. The tables have some common colors such as Red and Green.
The following statement joins the left table to the right table using the values in the color column:. As can be seen clearly from the result, the inner join returns rows from the left table that match with the rows from the right table. The following statement joins the left table with the right table using a left join or a left outer join :.
The left join returns all rows from the left table with the matching rows if available from the right table. Oracle Equi Joins retrieves the matching column values of the multiple tables. The below diagram represents the visual representation of the equijoin, as in the diagram the shaded area return as the result of the Oracle Equi Join. Another type of joins is an outer join which returns a result of an inner join plus all the rows from one table for which the join condition is not true.
The Left Outer Join returns contain all rows from the LEFT table according to the specified in ON condition and from the other table only those rows where the joined condition is true.
In self join the table uses twice in the FROM clause with the help of table aliases name. In other words, the self joins, join a table itself. The Oracle Self Join combines and returns rows of the table where the join condition is true. This above Oracle self JOIN example will return all rows from the employee table where the joined condition is true. You cannot compare a column with a subquery in the WHERE clause of any outer join, regardless which form you specify.
You can use outer joins to fill gaps in sparse data. Sparse data is data that does not have rows for all possible values of a dimension such as time or department.
For example, tables of sales data typically do not have rows for products that had no sales on a given date. Filling data gaps is useful in situations where data sparsity complicates analytic computation or where some data might be missed if the sparse data is queried directly. Oracle Database Data Warehousing Guide for a complete discussion of group outer joins and filling gaps in sparse data.
If you do not, then Oracle Database will return only the rows resulting from a simple join, but without a warning or error to advise you that you do not have the results of an outer join. For example, the following statement is not valid:. Otherwise Oracle returns only the results of a simple join. In previous releases of Oracle Database, in a query that performed outer joins of more than two pairs of tables, a single table could be the null-generated table for only one other table. Beginning with Oracle Database 12 c , a single table can be the null-generated table for multiple tables.
For example, the following statement is allowed in Oracle Database 12 c :. In this example, B , the null-generated table, is outer-joined to two tables, A and D.
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