在Oracle SQL Developer中联接表

Question

I have four tables that I want to join and display the output all together. I'm not sure how the syntax works for Oracle SQL Developer. I know this is an easy question for a programmer and I was hoping someone can make a suggestion on how the code might look like.

The tables are:

JNL1
JNL2
JNL3
JNL4

The key that is common between all four of these tables is ItemID .

How would the query look like? Thanks

Answer 1

It really depends on what kind of join you want (outer or not) but you can use default SQL syntax.

For example, joining without the JOIN keyword:

select * from JNL1, JNL2, JNL3, JNL4,
where 
JNL1.ItemID = JNL2.ItemID AND
JNL2.ItemID = JNL3.ItemID AND
JNL3.ItemID = JNL4.ItemID;

Additionally you can make use of multiple INNER JOINS eg

SELECT whatever
  FROM JNL1 AS a
INNER 
  JOIN JNL2 AS b
    ON b.ItemID = a.ItemID
INNER 
  JOIN JNL2 AS c
     ON c.ItemID = b.ItemID
INNER 
  JOIN JNL2 AS d
     ON d.ItemID = c.ItemID

Answer 2

It works in Oracle as it would in other DB engines :

SELECT *
FROM JNL1 j1
INNER JOIN JNL2 j2 ON j1.ItemID = j2.ItemID
INNER JOIN JNL3 j3 ON j1.ItemID = j3.ItemID
INNER JOIN JNL4 j4 ON j1.ItemID = j4.ItemID

One typical Oracle syntax exists when you want to LEFT JOIN :

Standard SQL:

SELECT *
FROM JNL1 j1
LEFT JOIN JNL2 j2 ON j1.ItemID = j2.ItemID
LEFT JOIN JNL3 j3 ON j1.ItemID = j3.ItemID
LEFT JOIN JNL4 j4 ON j1.ItemID = j4.ItemID

is equivalent to this Oracle syntax:

SELECT *
FROM JNL1 j1,
JNL2 j2,
JNL3 j3,
JNL4 j4,
WHERE j1.ItemID=j2.ItemID(+)
AND j1.ItemID=j3.ItemID(+)
AND j1.ItemID=j4.ItemID(+)

Answer 3

If you want to output the rows from all the tables, and if the columns are the same in each table, UNION ALL will work. Note that this isn't a JOIN in the database sense, though it kind of is in an English sense:

SELECT * FROM JNL1
UNION ALL SELECT * FROM JNL2
UNION ALL SELECT * FROM JNL3
UNION ALL SELECT * FROM JNL4

You can add an ORDER BY to the end if needed.

UNION is similar to UNION ALL , but it will omit rows where all of the values are duplicates.