在引入在线 DDL之前，将多个 DDL 操作合并到单个ALTER TABLE语句中是一种常见做法。由于每个ALTER TABLE语句都涉及复制和重建表，同时对同一个表进行多个更改更为高效，因为这些更改都可以通过一次表重建操作完成。缺点是涉及 DDL 操作的 SQL 代码更难维护，也不容易在不同的脚本中重用。如果每次特定的更改都不同，您可能需要为每个略有不同的情况构造一个新的复杂ALTER TABLE语句。Before the introduction of online DDL, it was common practice to combine many DDL operations into a single ALTER TABLE statement. Because each ALTER TABLE statement involved copying and rebuilding the table, it was more efficient to make several changes to the same table at once, since those changes could all be done with a single rebuild operation for the table. The downside was that SQL code involving DDL operations was harder to maintain and to reuse in different scripts. If the specific changes were different each time, you might have to construct a new complex ALTER TABLE for each slightly different scenario.

对于可以在线执行的 DDL 操作，您可以将它们拆分为单独的ALTER TABLE语句，以便于脚本编写和维护，同时不影响效率。例如，您可以将一个复杂的语句拆分为：For DDL operations that can be done online, you can separate them into individual ALTER TABLE statements for easier scripting and maintenance, without sacrificing efficiency. For example, you might take a complicated statement such as:

ALTER TABLE t1 ADD INDEX i1(c1), ADD UNIQUE INDEX i2(c2),
  CHANGE c4_old_name c4_new_name INTEGER UNSIGNED;

并将其分解为可以独立测试和执行的更简单的部分，例如：and break it down into simpler parts that can be tested and performed independently, such as:

ALTER TABLE t1 ADD INDEX i1(c1);
ALTER TABLE t1 ADD UNIQUE INDEX i2(c2);
ALTER TABLE t1 CHANGE c4_old_name c4_new_name INTEGER UNSIGNED NOT NULL;

您仍然可以使用多部分的ALTER TABLE语句来处理：You might still use multi-part ALTER TABLE statements for: