大一小白提问: MYSQL 中利用 NOT EXIST 实现关系除法运算

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大一小白一枚, 想请教一下各位大佬们 MYSQL 中实现关系除法的双重 NOT EXIST 怎么理解呀?对着课本看了好久也没看懂, 谷歌上查也不太能理解比如这个, (empskills 表是员工技能表, skills 是技能表) SELECT DISTINCT emp FROM EmpSkills AS ES1 WHERE NOT EXISTS (SELECT * FROM skills WHERE NOT EXISTS (SELECT * FROM empSkills AS ES2 WHERE ES2.emp = ES1.emp AND ES2.skill = skills.skill)); 不知道怎么解释

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select empskills emp not