"ORA-00913: too many values" error when I run SQL query

Question

I'm getting a "too many values" error and I'm not sure how to approach it. The problem starts at the first nested SELECT

SELECT p.name, p.phone_number 
    FROM person p WHERE 
    (SELECT ks.title, ks.ks_code 
    FROM required_skills rs JOIN knowledge_skills ks USING (ks_code) 
    WHERE rs.pos_code = 'CS1') 
    IN (SELECT ks.title, ks.ks_code 
        FROM courses_taken ct  JOIN course_learning_units clu 
        USING (c_code)  JOIN learning_unit_skills lus USING (lu_code) 
        JOIN knowledge_skills ks USING (ks_code))

I looked up info about this error and I believe it's because the subquery in the WHERE returns too many columns. The table data is below. But how can I whittle that down? I appreciate any pointers, Thanks

required_skills

POS_CODE    VARCHAR2(10 BYTE)   Yes     1   
KS_CODE VARCHAR2(10 BYTE)   Yes     2   

learning_unit_skills

LU_CODE VARCHAR2(10 BYTE)   Yes     1   
KS_CODE VARCHAR2(10 BYTE)   Yes     2

person

PER_ID  NUMBER(38,0)    No      1   
NAME    VARCHAR2(255 BYTE)  Yes     2   
STREET  VARCHAR2(255 BYTE)  Yes     3   
CITY    VARCHAR2(60 BYTE)   Yes     4   
ZIP_CODE    VARCHAR2(60 BYTE)   Yes     5   
PHONE_NUMBER    VARCHAR2(255 BYTE)  Yes     6   
GENDER  VARCHAR2(6 BYTE)    Yes     7   

course learning units

C_CODE  VARCHAR2(10 BYTE)   Yes     1   
LU_CODE VARCHAR2(10 BYTE)   Yes     

courses taken

C_CODE  VARCHAR2(10 BYTE)   Yes     1   
PER_ID  NUMBER(38,0)    Yes     2   

Answer 1

A quick rewrite:

SELECT p.name, p.phone_number 
    FROM person p WHERE EXISTS
    (SELECT * 
    FROM required_skills rs JOIN knowledge_skills ks USING (ks_code) 
    WHERE rs.pos_code = 'CS1'
      AND (ks.title, ks.ks_code)
    IN (SELECT ks.title, ks.ks_code 
        FROM courses_taken ct  JOIN course_learning_units clu 
        USING (c_code)  JOIN learning_unit_skills lus USING (lu_code) 
        JOIN knowledge_skills ks USING (ks_code))
    )

or (the joins will probably need corrections):

SELECT p.name, p.phone_number 
    FROM person p WHERE EXISTS
    (SELECT * 
     FROM required_skills rs JOIN knowledge_skills ks USING (ks_code) 
        JOIN ( courses_taken ct  JOIN course_learning_units clu 
               USING (c_code)  JOIN learning_unit_skills lus USING (lu_code) 
              ) USING (ks_code)
     WHERE rs.pos_code = 'CS1'
    )

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Problem is that in your code there is no relation to table person. I guess you should have one in the subqueries - making them correlated.