01 - MySQL EBNF

SQL keywords are not case-sensitive and may be written in any lettercase. This manual uses uppercase

[ ]

Square brackets (“[” and “]”) indicate optional words or clauses
```
DROP TABLE [IF EXISTS] tbl_name
```
例如清空表的时候可以选择使用 IF EXISTS
|

When a syntax element consists of a number of alternatives, the alternatives are separated by vertical bars (“|”).
```
TRIM([[BOTH | LEADING | TRAILING] [remstr] FROM] str)
```
例如 TRIM 函数如果有 3 个入参第一个入参可以是 BOTH
{ }

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