Do not insert duplicates into mysql in python [duplicate]

I started by googling and found the article How to write INSERT if NOT EXISTS queries in standard SQL which talks about mutex tables.

I have a table with ~14 million records. If I want to add more data in the same format, is there a way to ensure the record I want to insert does not already exist without using a pair of queries (i.e., one query to check and one to insert is the result set is empty)?

Does a unique constraint on a field guarantee the insert will fail if it's already there?

It seems that with merely a constraint, when I issue the insert via PHP, the script croaks.

Use INSERT IGNORE INTO table.

There's also INSERT … ON DUPLICATE KEY UPDATE syntax, and you can find explanations in 13.2.6.2 INSERT ... ON DUPLICATE KEY UPDATE Statement.

Post from bogdan.org.ua according to Google's webcache:

18th October 2007

To start: as of the latest MySQL, syntax presented in the title is not possible. But there are several very easy ways to accomplish what is expected using existing functionality.

There are 3 possible solutions: using INSERT IGNORE, REPLACE, or INSERT … ON DUPLICATE KEY UPDATE.

Imagine we have a table:

CREATE TABLE `transcripts` (
`ensembl_transcript_id` varchar(20) NOT NULL,
`transcript_chrom_start` int(10) unsigned NOT NULL,
`transcript_chrom_end` int(10) unsigned NOT NULL,
PRIMARY KEY (`ensembl_transcript_id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1;

Now imagine that we have an automatic pipeline importing transcripts meta-data from Ensembl, and that due to various reasons the pipeline might be broken at any step of execution. Thus, we need to ensure two things:

1. repeated executions of the pipeline will not destroy our > database

2. repeated executions will not die due to ‘duplicate > primary key’ errors.

Method 1: using REPLACE

It’s very simple:

REPLACE INTO `transcripts`
SET `ensembl_transcript_id` = 'ENSORGT00000000001',
`transcript_chrom_start` = 12345,
`transcript_chrom_end` = 12678;

If the record exists, it will be overwritten; if it does not yet exist, it will be created. However, using this method isn’t efficient for our case: we do not need to overwrite existing records, it’s fine just to skip them.

Method 2: using INSERT IGNORE Also very simple:

INSERT IGNORE INTO `transcripts`
SET `ensembl_transcript_id` = 'ENSORGT00000000001',
`transcript_chrom_start` = 12345,
`transcript_chrom_end` = 12678;

Here, if the ‘ensembl_transcript_id’ is already present in the database, it will be silently skipped (ignored). (To be more precise, here’s a quote from MySQL reference manual: “If you use the IGNORE keyword, errors that occur while executing the INSERT statement are treated as warnings instead. For example, without IGNORE, a row that duplicates an existing UNIQUE index or PRIMARY KEY value in the table causes a duplicate-key error and the statement is aborted.”.) If the record doesn’t yet exist, it will be created.

This second method has several potential weaknesses, including non-abortion of the query in case any other problem occurs (see the manual). Thus it should be used if previously tested without the IGNORE keyword.

Method 3: using INSERT … ON DUPLICATE KEY UPDATE:

Third option is to use INSERT … ON DUPLICATE KEY UPDATE syntax, and in the UPDATE part just do nothing do some meaningless (empty) operation, like calculating 0+0 (Geoffray suggests doing the id=id assignment for the MySQL optimization engine to ignore this operation). Advantage of this method is that it only ignores duplicate key events, and still aborts on other errors.

As a final notice: this post was inspired by Xaprb. I’d also advise to consult his other post on writing flexible SQL queries.

Solution:

INSERT INTO `table` (`value1`, `value2`) 
SELECT 'stuff for value1', 'stuff for value2' FROM DUAL 
WHERE NOT EXISTS (SELECT * FROM `table` 
      WHERE `value1`='stuff for value1' AND `value2`='stuff for value2' LIMIT 1) 

Explanation:

The innermost query

SELECT * FROM `table` 
      WHERE `value1`='stuff for value1' AND `value2`='stuff for value2' LIMIT 1

used as the WHERE NOT EXISTS-condition detects if there already exists a row with the data to be inserted. After one row of this kind is found, the query may stop, hence the LIMIT 1 (micro-optimization, may be omitted).

The intermediate query

SELECT 'stuff for value1', 'stuff for value2' FROM DUAL

represents the values to be inserted. DUAL refers to a special one row, one column table present by default in all Oracle databases (see https://en.wikipedia.org/wiki/DUAL_table). On a MySQL-Server version 5.7.26 I got a valid query when omitting FROM DUAL, but older versions (like 5.5.60) seem to require the FROM information. By using WHERE NOT EXISTS the intermediate query returns an empty result set if the innermost query found matching data.

The outer query

INSERT INTO `table` (`value1`, `value2`) 

inserts the data, if any is returned by the intermediate query.