SQLachemy的查询顺序和mysql的查询数据原理分析（一致性规则非常重要）

清疚 2021-09-18 01:38 157阅读 0赞

### **mySQL正确的语法执行顺序(官方查询规则：**[https://dev.mysql.com/doc/refman/8.0/en/select.html][https_dev.mysql.com_doc_refman_8.0_en_select.html]**)** ###

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MzM0MzE0NA_size_16_color_FFFFFF_t_70][]

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MzM0MzE0NA_size_16_color_FFFFFF_t_70 1][]

#mysql例子：
    
    SELECT DISTINCT * from gift WHERE launched=0 GROUP BY gift.isbn
    
    #正确编写顺序...
    select ... from... where.... group by... having... order by..

## ##

### **SQLachemy用函数的方式替换了这些语法规则，但是原理顺序和mysql是一样的（必须遵循一致性查询规则）** ###

#SQLachemy的执行顺序（同mysql） 
    #select distinct（DISTINCT去重）----all(*最终查询的结果)   
    #from----filter_by（where）----group_by（分组）----order_by（排序）-----limit（显示条数）
    
    
    def query_all_gift():
            return Gift.query.filter_by(launched=False).group_by(Gift.isbn).order_by(
                Gift.create_time).limit(30).distinct().all()

**SQLachemy的查询Query函数大全去官方查找：**[https://docs.sqlalchemy.org/en/latest/orm/query.html\#sqlalchemy.orm.query.Query.limit][https_docs.sqlalchemy.org_en_latest_orm_query.html_sqlalchemy.orm.query.Query.limit]

**Colum列属性方法参考**：[https://docs.sqlalchemy.org/en/latest/core/sqlelement.html?highlight=desc\#sqlalchemy.sql.expression.ColumnElement.desc][https_docs.sqlalchemy.org_en_latest_core_sqlelement.html_highlight_desc_sqlalchemy.sql.expression.ColumnElement.desc]

[https_dev.mysql.com_doc_refman_8.0_en_select.html]: https://dev.mysql.com/doc/refman/8.0/en/select.html
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MzM0MzE0NA_size_16_color_FFFFFF_t_70]: /images/20210812/610b9378313149dca2f947300993e1c7.png
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MzM0MzE0NA_size_16_color_FFFFFF_t_70 1]: /images/20210812/6738ee4093074e5dbe8e1c5f2804d55f.png
[https_docs.sqlalchemy.org_en_latest_orm_query.html_sqlalchemy.orm.query.Query.limit]: https://docs.sqlalchemy.org/en/latest/orm/query.html#sqlalchemy.orm.query.Query.limit
[https_docs.sqlalchemy.org_en_latest_core_sqlelement.html_highlight_desc_sqlalchemy.sql.expression.ColumnElement.desc]: https://docs.sqlalchemy.org/en/latest/core/sqlelement.html?highlight=desc#sqlalchemy.sql.expression.ColumnElement.desc