原文链接
首先经过token进行切词，然后通过ast node来进行组织
可以发现，ast parser 的时候，预先构造好状态空间，比如 select 的状态空间:
- expression list
- from tables
- where
- group by
- with …
- order by
- limit
在一个状态空间內，还可以根据 parse 返回的 bool 判断是否继续进入子状态空间，一直递归解析出整个 ast。
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| bool ParserSelectQuery::parseImpl(Pos & pos, ASTPtr & node, Expected & expected)
{
auto select_query = std::make_shared<ASTSelectQuery>();
node = select_query;
ParserKeyword s_select("SELECT");
ParserKeyword s_distinct("DISTINCT");
ParserKeyword s_from("FROM");
ParserKeyword s_prewhere("PREWHERE");
ParserKeyword s_where("WHERE");
ParserKeyword s_group_by("GROUP BY");
ParserKeyword s_with("WITH");
ParserKeyword s_totals("TOTALS");
ParserKeyword s_having("HAVING");
ParserKeyword s_order_by("ORDER BY");
ParserKeyword s_limit("LIMIT");
ParserKeyword s_settings("SETTINGS");
ParserKeyword s_by("BY");
ParserKeyword s_rollup("ROLLUP");
ParserKeyword s_cube("CUBE");
ParserKeyword s_top("TOP");
ParserKeyword s_with_ties("WITH TIES");
ParserKeyword s_offset("OFFSET");
ParserNotEmptyExpressionList exp_list(false);
ParserNotEmptyExpressionList exp_list_for_with_clause(false);
ParserNotEmptyExpressionList exp_list_for_select_clause(true);
...
}
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