mysql insert if value not exist in another table

Question

I have two tables that store value as VARCHAR.
I'm populating table and I want just insert values in one of tables if they are not exist in other table.
Something like:

INSERT IF IS EMPTY(SELECT * FROM t1 where v='test') INTO t2 (v) VALUES ('test')

How Can I do that?

Solution 1

You need to use some type of INSERT...SELECT query.

Update (after clarification): For example, here is how to insert a row in t2 if a corresponding row do not already exist in t1:

INSERT INTO t2 (v)
  SELECT temp.candidate
  FROM (SELECT 'test' AS candidate) temp
  LEFT JOIN t1 ON t1.v = temp.candidate
  WHERE t1.v IS NULL

To insert multiple rows with the same query, I 'm afraid there is nothing better than

INSERT INTO t2 (v)
  SELECT temp.candidate
  FROM (
      SELECT 'test1' AS candidate
      UNION SELECT 'test2'
      UNION SELECT 'test3' -- etc
  ) temp
  LEFT JOIN t1 ON t1.v = temp.candidate
  WHERE t1.v IS NULL

Original answer

For example, this will take other_column from all rows from table1 that satisfy the WHERE clause and insert rows into table2 with the values used as column_name. It will ignore duplicate key errors.

INSERT IGNORE INTO table2 (column_name)
  SELECT table1.other_column
  FROM table1 WHERE table1.something == 'filter';

Solution 2

for insertion of Multiple columns you can try this.

INSERT INTO table_1 (column_id,column_2,column_3,column_4,column_5)
 SELECT
    table_2.*
FROM
    table_2
LEFT JOIN table_1 ON table_1.column_id = table_2.column_id
WHERE
    table_1.column_id IS NULL;

Solution 3

INSERT IGNORE INTO table_1(col1,col2,col3,col4)
  SELECT table_2.*
  FROM table_2;

The above statement does the perfect job in your scenario.

1. Line 1: States the table where you want the data to be replicated whilst ignoring any key constraints.
2. Line 2: selects the table and columns where data should come from
3. Line 3: states the table from where data is to be replicated from

Comments

[Ariyan]

But this inserts into table2 and gets values from table1 ! While I want to insert if the value doesn't exists in table1! Please See my edit on question, thanks

[Waihon Yew]

@4r1y4n: If the value doesn't exist in table1 because you say so, and it doesn't exist in table2 because we are trying to insert it there, then where does it exist?

[Ariyan]

In SQL query itself (See my psudo query in question - 'test' is the value)! I just want insert value only in one of tables and not both (t1 XOR t2)

[Ariyan]

This is PHP and not pure MySQL! of course this works but I want to do it using an MySQL query

[yoeriboven]

Okay, with my last sentence I meant it'd be better to use PDO or MySQLi. I see I didn't get your question and unfortunately don't know the answer. Sorry!

[BurninLeo]

As of MySQL 8.0.19 you won't need a temporary table to define the data, but use VALUE instead (dev.mysql.com/doc/refman/8.0/en/values.html).

[BurninLeo]

Practical note: This works fine to insert a small number of values. But when inserting a few hundret values, and checking this against a few 100k existing values, a 200x250.000 JOIN makes the query painfully slow.