用表 B 中的类似列填充表 A 上的现有列的最佳 SQL 查询是什么？

Question

Let's say I have an existing table A with a column called contact_name and a ID column id as the primary key. All the rows in A have the name value as "NULL" right now.

Another table B has different columns, but one of which is contact_name , and another is ref_id . Each ref_id in B corresponds to a value of id in A , and there may be multiple rows in B that share the same value for ref_id (meaning they all correspond to a single entry in A ).

Let me set up an example:

Table A

id | contact_name
1  | [NULL]
2  | [NULL]

Table B

ref_id | contact_name
1      | "John"
2      | "Helen"
2      | "Alex"

Note there are theoretically other values in each table but for the sake of brevity I'm just showing the values I'm interested in using.

I want to populate contact_name in table A with the first entry of the corresponding contact_name in B , where B. (first) ref_id = A.id , without adding any rows or editing the rest of the rows in either table. That is, I want A in my example to now be:

id | contact_name
1  | "John"
2  | "Helen"

Again, note how the first contact_name value, "Helen", in B is selected, not any other subsequent one, like "Alex".

Answer 1

The MERGE function is made for this. Give it a try:

MERGE INTO TABLEA a USING (
    SELECT b.REF_ID, b.CONTACT_NAME
    FROM TABLEB) b
ON (a.ID = b.REF_ID)
WHEN MATCHED THEN
    UPDATE SET
        a.CONTACT_NAME = b.CONTACT_NAME
    WHERE a.CONTACT_NAME IS null;