project_skills.skill_id = 7) as s on projects.id = s.skill_id;| count(*) |select count(*) from projects join(select project_skills.skill_id from project_skills where project_skills.skill_id= 7) as s on projects.i
包含ID(PK)、PID(FK)、Skill(FK)和SkillLevel(FK)的Person_Skill。包含ID(主键)和SkillLabel的技能。每种技能都有0到7的等级Select [Skill].SkillLabel[Person_Skill] ON [dbo].[Person_Skill].PID= Person.ID)
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