In this regard, the query is similar to a leftouterjoin in SQL.
从这种意义上说,查询与 SQL 中的左外连接相似。
2
Figure 8 shows that the leftouterjoin operations are cascaded if a rule USES more than one reference column.
图8展示了如果规则使用了多个参考列,那么左外部连接就进行级联。
3
The combined record is much easier to use-you only have to check the PICTURE field to see if it's not null (which was the result of the LEFTOUTERJOIN).