WHICH one should I use in this case
I am a novice in SQL, so this may be a simple.
I have 3 tables PARENT, CHILD and PARENT_CHILD
PARENT has a column P_NAME
CHILD has 2 columns ane C_NAME C_ID
PARENT_CHILD has 2 columns P_NAME and C_ID, and lists these parents who have children.
What query use to list all the parents and the children they have, but also includes parents without children
Kind regards
Hello
a02d218c-478C-40DD-9b58-b5324075b244 wrote:
... What query is used to list all of the parents and the children they have, but also includes parents without children...
It is an outer join.
An inner join includes data of table x with the corresponding rows in the table, only when there are matching rows.
Outer join contains data from table x with lines of table y (if there is), but it shows data from x without worrying.
So in this case, you want outside parent joined parent_child; This means that the result set will have a few lines that don't match anything in the child, then, when you join child on this result set, which should also do an outer join:
SELECT p.p_name
c.c_id
c.c_name
PARENTS p
LEFT OUTER JOIN parent_child pc ON pc.p_name = p.p_name
Class c of child ON c.c_id = pc.c_id LEFT OUTER JOIN
ORDER BY p.p_name - or else
;
This type of outer join in cascade is very common. If the lines of pc may or may not be present, because the pc is external attached, then each join involving the pc must be an outer join, otherwise the efffect will be identical to an inner join.
I recommend the syntax of ANSI join (with JOIN keywords and more) for all joins, but especially for outer joins. It is easier to understand, it eliminates accidental cross joins, and reduces the need for you to subqueries.
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