SQL查询报错：Not unique table/alias: 'ep' 问题求助

错误原因 #

你写的SQL同时混用了旧版逗号分隔表和JOIN连接两种语法，还重复给同一个表定义了相同别名：

• 先是在FROM里用逗号列出了endpoints ep、entitlement_types et、entitlement_values ev三张表
• 后面又用JOIN语句再次引入endpoints ep和entitlement_types et，导致ep、et这两个别名被重复绑定，数据库无法识别到底对应哪次引用的表

修正后的SQL #

SELECT 
    ep.endpointname,
    ep.displayname,
    ep.description,
    et.Customproperty1_Label,
    ev.Customproperty1,
    et.Customproperty2_Label,
    ev.Customproperty2,
    et.Customproperty3_Label,
    ev.Customproperty3,
    et.Customproperty4_Label,
    ev.Customproperty4,
    et.Customproperty5_Label,
    ev.Customproperty5,
    et.Customproperty6_Label,
    ev.Customproperty6,
    et.Customproperty7_Label,
    ev.Customproperty7,
    et.Customproperty8_Label,
    ev.Customproperty8,
    et.Customproperty9_Label,
    ev.Customproperty9,
    et.Customproperty10_Label,
    ev.Customproperty10,
    et.Customproperty11_Label,
    ev.Customproperty11,
    et.Customproperty12_Label,
    ev.Customproperty12,
    et.Customproperty13_Label,
    ev.Customproperty13,
    et.Customproperty14_Label,
    ev.Customproperty14,
    et.Customproperty15_Label,
    ev.Customproperty15,
    et.Customproperty16_Label,
    ev.Customproperty16,
    et.Customproperty17_Label,
    ev.Customproperty17,
    et.Customproperty18_Label,
    ev.Customproperty18,
    et.Customproperty19_Label,
    ev.Customproperty19,
    et.Customproperty20_Label,
    ev.Customproperty20,
    et.Customproperty21_Label,
    ev.Customproperty21,
    et.Customproperty22_Label,
    ev.Customproperty22,
    et.Customproperty23_Label,
    ev.Customproperty23,
    et.Customproperty24_Label,
    ev.Customproperty24,
    et.Customproperty25_Label,
    ev.Customproperty25,
    et.Customproperty26_Label,
    ev.Customproperty26,
    et.Customproperty27_Label,
    ev.Customproperty27,
    et.Customproperty28_Label,
    ev.Customproperty28,
    et.Customproperty29_Label,
    ev.Customproperty29,
    et.Customproperty30_Label,
    ev.Customproperty30,
    et.Customproperty31_Label,
    ev.Customproperty31,
    et.Customproperty32_Label,
    ev.Customproperty32,
    et.Customproperty33_Label,
    ev.Customproperty33,
    et.Customproperty34_Label,
    ev.Customproperty34,
    et.Customproperty35_Label,
    ev.Customproperty35,
    et.Customproperty36_Label,
    ev.Customproperty36,
    et.Customproperty37_Label,
    ev.Customproperty37,
    et.Customproperty38_Label,
    ev.Customproperty38,
    et.Customproperty39_Label,
    ev.Customproperty39,
    et.Customproperty40_Label,
    ev.Customproperty40
FROM endpoints ep
INNER JOIN entitlement_types et 
    ON ep.endpointkey = et.endpointkey
INNER JOIN entitlement_values ev 
    ON et.entitlementtypekey = ev.entitlementtypekey

修正说明 #

1. 统一使用标准的INNER JOIN语法，从endpoints表开始依次关联另外两张表
2. 每个表只定义一次别名，确保ep、et、ev唯一对应各自的表
3. 明确每个JOIN的连接条件，避免产生无意义的笛卡尔积

内容的提问来源于stack exchange，提问作者jessie.szprejda