# Clustered Vs. Non-Clustered Tables (Continue...)

+ Add or delete nonclustered indexes
  + TiDB supports adding or removing non-clustered indexes after a table is created. At this point, you can choose to explicitly specify the NONCLUSTERED keyword or omit the keyword
+ Add or delete clustered indexes
  + Currently TiDB does not support adding or deleting clustered indexes after table creation, nor does it support mutual conversion between clustered and non-clustered indexes

```
ALTER TABLE t ADD PRIMARY KEY(b, a) NONCLUSTERED;
ALTER TABLE t ADD PRIMARY KEY(b, a); -- 不指定关键字，则为非聚簇索引
ALTER TABLE t DROP PRIMARY KEY;
ALTER TABLE t DROP INDEX `PRIMARY`;
```
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