I tried writing query using exists, but no success so far. Searching hasn't helped so far.
If you attempt to alter a table that does not exist, the query will fail with an error: Table 'database.table' doesn't exist
MySQL does support ALTER IGNORE TABLE, but that only turns errors into warnings if you're attempting to create a unique index while there are values in the table that violate that index.
If you would like to make sure that you do not produce any database queries, I would suggest ensuring the table's existence using SHOW TABLES LIKE 'tablename' before running your ALTER TABLE query.
Related
My MySQL table "crashed" after updating column types. I changed VARCHAR to INT and added some new columns too.
After that, when I'd like to view the table entries, every software just keeps loading and crashing... I can't even make any queries to the table. All I can do is look at the list of columns of the table. (I've tried with PhpMyAdmin, HeidiSQL and MySQL Workbench). Changes are made with HeidiSQL.
What should I do? This is the first time this happens and I've been using HeidiSQL for a long time.
You have changed VARCHAR to INT in the table schema. At first sight we are correct, just change alter the column and mysql server do as without showing any error But when you alter the table, mysql server have to do lot of work. first store the records temporary and recreate the schema and insert the record. If your table have many records then it is tough to alter. So either truncate table and alter the schema and reinsert or create other table and rename it latter.
In my opinion the solution is create table as you want schema.
Insert the record in new table from table.
eg- insert into new_table (col1, col2) select col1, col2 from old_table;
drop the old table
drop table old_table
rename the new table with old table
rename new_table to old_table
I'm having an issue with finding and deleting duplicate records, I have a table with IDs called CallDetailRecordID which I need to scan and delete records, the reason there are duplicates is that I'm exporting data to special arching engine works with MySQL and it doesn't support indexing.
I tried using "Select DISTINCT" but it dosn't work, is there is another way? I'm hoping I can create a store procedure and have it run weekly to perform clean up.
your help is highly appreciated.
Thank you
CREATE TABLE tmp_table LIKE table
INSERT INTO tmp_table (SELECT * FROM table GROUP BY CallDetailRecordID)
RENAME table TO old_table
RENAME tmp_table to table
Drop the old table if you want, add a LOCK TABLES statement at the beginning to avoid lost inserts.
I am using MySQL v5.1.
I would like to create index on a table by executing the following SQL statement:
CREATE INDEX index_name
ON table_name (column_name)
But, I wan to firstly check if the index on that column has already been created or not, if not, create it (otherwise do not create). What is the SQL syntax for this?
Mysql doesn't have IF NOT EXISTS for CREATE INDEX. You can work it out by querying information_schema.statistics table. Take a look here, there is an example of stored procedure that does what you are looking for (search for "CREATE INDEX IF NOT EXISTS" on the page)
You want SHOW INDEX.
To get all the indexes on a table:
SHOW INDEX FROM table_name
MySQL allows you to add a WHERE clause to limit the results as well.
Lock the table while you're checking to see if the index exists (and if it doesn't exist, creating the index) so that another process doesn't create the index right after you've checked for it but before you've created it yourself.
Is it possible to use the LIKE statement on ALTER TABLE similar to CREATE TABLE in MySQL?
Eg. 'CREATE TABLE db.tbl1 LIKE db.tbl2'
This clones a database table's structure. I want to alter an existing table with the same columns but to pick up the primary keys of another table.
I was thinking of something like 'ALTER TABLE db.tbl1 LIKE db.tbl2' but this throws back an error.
Any ideas?
Thanks
I required a similar thing and settled to use the following procedure:
ALTER TABLE tbl1 RENAME tbl1_old;
CREATE TABLE tbl1 LIKE tbl2;
INSERT INTO tbl1 SELECT * FROM tbl1_old;
DROP TABLE tbl1_old;
Altough this is not a single statement it should do the job. Only problem could be different index settings (UNIQUE, etc.) which cause errors when doing the "INSERT INTO" of the original table contents.
Everything ALTER TABLE can do is explained here.
As you can see importing indexes from another table is not mentioned. You could probably do that with some clever information_schema querying, but I don't think it would be worth the cost.
It seems you can't.
I have a table that has 170,002,225 rows with about 35 columns and two indexes. I want to add a column. The alter table command took about 10 hours. Neither the processor seemed busy during that time nor were there excessive IO waits. This is on a 4 way high performance box with tons of memory.
Is this the best I can do? Is there something I can look at to optimize the add column in tuning of the db?
I faced a very similar situation in the past and i improve the performance of the operation in this way :
Create a new table (using the structure of the current table) with the new column(s) included.
execute a INSERT INTO new_table (column1,..columnN) SELECT (column1,..columnN) FROM current_table;
rename the current table
rename the new table using the name of the current table.
ALTER TABLE in MySQL is actually going to create a new table with new schema, then re-INSERT all the data and delete the old table. You might save some time by creating the new table, loading the data and then renaming the table.
From "High Performance MySQL book" (the percona guys):
The usual trick for loading MyISAM table efficiently is to disable keys, load the data and renalbe the keys:
mysql> ALTER TABLE test.load_data DISABLE KEYS;
-- load data
mysql> ALTER TABLE test.load_data ENABLE KEYS;
Well, I would recommend using latest Percona MySQL builds plus since there is the following note in MySQL manual
In other cases, MySQL creates a
temporary table, even if the data
wouldn't strictly need to be copied.
For MyISAM tables, you can speed up
the index re-creation operation (which
is the slowest part of the alteration
process) by setting the
myisam_sort_buffer_size system
variable to a high value.
You can do ALTER TABLE DISABLE KEYS first, then add column and then ALTER TABLE ENABLE KEYS. I don't see anything can be done here.
BTW, can't you go MongoDB? It doesn't rebuild anything when you add column.
Maybe you can remove the index before alter the table because what is take most of the time to build is the index?
Combining some of the comments on the other answers, this was the solution that worked for me (MySQL 5.6):
create table mytablenew like mytable;
alter table mytablenew add column col4a varchar(12) not null after col4;
alter table mytablenew drop index index1, drop index index2,...drop index indexN;
insert into mytablenew (col1,col2,...colN) select col1,col2,...colN from mytable;
alter table mytablenew add index index1 (col1), add index index2 (col2),...add index indexN (colN);
rename table mytable to mytableold, mytablenew to mytable
On a 75M row table, dropping the indexes before the insert caused the query to complete in 24 minutes rather than 43 minutes.
Other answers/comments have insert into mytablenew (col1) select (col1) from mytable, but this results in ERROR 1241 (21000): Operand should contain 1 column(s) if you have the parenthesis in the select query.
Other answers/comments have insert into mytablenew select * from mytable;, but this results in ERROR 1136 (21S01): Column count doesn't match value count at row 1 if you've already added a column.