FPMLADMIN.COMMENTX, which indexes the relational COMMENT column.
它索引关系comment列。
With this in mind, in Figure 2, note that one row has a null entry for the COMMENT column.
记住这些，在图2中，注意comment列的某一行有一个空的条目。
When you want to determine whether a given row includes a comment, you no longer need to check for a null value in the COMMENT column.
确定某一个行是否包含注释时，不再需要检查 COMMENT 列的 null 值。
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