What is the most efficient way to duplicate a row in an Sqlite3 database exactly except with an updated PrimaryKey?
Use an insert .. select where both the insert and select reference the same relation. Then the entire operation will occur as a single statement within SQLite code itself which cannot be beaten for "efficiency".
It will be easier with an auto-PK (just don't select the PK column), although an appropriate natural key can be assigned as well if such can be derived.
See SQLite INSERT SELECT Query Results into Existing Table?
Related
I am trying to delete column last_name from Persons using FMDB,
let query = "ALTER TABLE Persons DROP COLUMN last_name;"
try FMDBHelper.database.executeUpdate(query, values: nil)
But comes with error
DB Error: 1 "near "DROP": syntax error".
sqlite does not support DROP COLUMN in ALTER TABLE.
You can only rename tables and add columns.
If you need to remove columns, create a new table, copy the data there, drop the old table and rename the table to its intented name.
Reference: http://www.sqlite.org/lang_altertable.html
Please note that I flagged that your question could be duplicated, I will provide an answer to make it more clear.
I think that you are missing a point, which is: The FMDB is (as mentioned in their repo description):
This is an Objective-C wrapper around SQLite
Keep in mind that since FMDB is built on top of SQLite, it is not a limitation from the library itself; it is related to how SQLite ALTER TABLE works.
The SQLite ALTER TABLE statement is limited to rename a table and add a new column to the desired table:
SQLite supports a limited subset of ALTER TABLE. The ALTER TABLE
command in SQLite allows the user to rename a table or to add a new
column to an existing table.
http://www.sqlite.org/lang_altertable.html
For achieving what are you looking for:
You could check the answers of Delete column from SQLite table.
This is how I currently have my methods in my project to use sqlite queries to return NSObjects to be used in my iOS project:
in the launch of my application, each table is checked if it needs to be created
in the application, there is no DROP TABLE .. queries nor ALTER TABLE ..
My question is:
should I be checking if a table exists every time I'm going to create a sqlite3 query?
should i use CREATE TABLE IF NOT EXISTS .. vs checking if a table exists using a different query like:
SELECT name FROM sqlite_master WHERE type IN ('table','view') AND name NOT LIKE 'sqlite_%' UNION ALL SELECT name FROM sqlite_temp_master WHERE type IN ('table','view') ORDER BY 1;and iterating the names of the tables and checking if the table name exists?
I want to create the least amount of overhead. i am not using Core Data as well
Also, while updating records, i've noticed if i insert the name of a column, it takes the value of that column:
I got the data by 'MODEL.all' command in rails console
I want to put the column 'cgi_name' in the 3rd position when I run MODEL.all in the rails console
I use the postgres for my DB
How to get it ?
To answer your question directly, you'll have to move the columns at DB level
Currently, I only know MYSQL to support this functionality:
ALTER TABLE Employees CHANGE COLUMN empName empName VARCHAR(50) AFTER department;
Postgres, to my knowledge, does not support this functionality:
Many people new to postgresql often ask if it has support for altering
column positions within a table. Currently it does not; if you want to
change column positions, you must either recreate the table, or add
new columns and move data
In the view, you'll have to either manually display the columns, or create a helper method to cycle through them in an order of your choosing
Simple answer is YOU CANNOT
There is no way to re-order the column names to be displayed when you select using Model.all.
Otherwise, you can re-order this by selecting each column in the order you want.
Model.select("column1, column2, cgi_name, column4 etc..")
Hope it helps :)
I have a Join table in Rails which is just a 2 column table with ids.
In order to mass insert into this table, I use
ActiveRecord::Base.connection.execute("INSERT INTO myjointable (first_id,second_id) VALUES #{values})
Unfortunately this gives me errors when there are duplicates. I don't need to update any values, simply move on to the next insert if a duplicate exists.
How would I do this?
As an fyi I have searched stackoverflow and most the answers are a bit advanced for me to understand. I've also checked the postgresql documents and played around in the rails console but still to no avail. I can't figure this one out so i'm hoping someone else can help tell me what I'm doing wrong.
The closest statement I've tried is:
INSERT INTO myjointable (first_id,second_id) SELECT 1,2
WHERE NOT EXISTS (
SELECT first_id FROM myjointable
WHERE first_id = 1 AND second_id IN (...))
Part of the problem with this statement is that I am only inserting 1 value at a time whereas I want a statement that mass inserts. Also the second_id IN (...) section of the statement can include up to 100 different values so I'm not sure how slow that will be.
Note that for the most part there should not be many duplicates so I am not sure if mass inserting to a temporary table and finding distinct values is a good idea.
Edit to add context:
The reason I need a mass insert is because I have a many to many relationship between 2 models where 1 of the models is never populated by a form. I have stocks, and stock price histories. The stock price histories are never created in a form, but rather mass inserted themselves by pulling the data from YahooFinance with their yahoo finance API. I use the activerecord-import gem to mass insert for stock price histories (i.e. Model.import columns,values) but I can't type jointable.import columns,values because I get the jointable is an undefined local variable
I ended up using the WITH clause to select my values and give it a name. Then I inserted those values and used WHERE NOT EXISTS to effectively skip any items that are already in my database.
So far it looks like it is working...
WITH withqueryname(first_id,second_id) AS (VALUES(1,2),(3,4),(5,6)...etc)
INSERT INTO jointablename (first_id,second_id)
SELECT * FROM withqueryname
WHERE NOT EXISTS(
SELECT first_id FROM jointablename WHERE
first_id = 1 AND
second_id IN (1,2,3,4,5,6..etc))
You can interchange the Values with a variable. Mine was VALUES#{values}
You can also interchange the second_id IN with a variable. Mine was second_id IN #{variable}.
Here's how I'd tackle it: Create a temp table and populate it with your new values. Then lock the old join values table to prevent concurrent modification (important) and insert all value pairs that appear in the new table but not the old one.
One way to do this is by doing a left outer join of the old values onto the new ones and filtering for rows where the old join table values are null. Another approach is to use an EXISTS subquery. The two are highly likely to result in the same query plan once the query optimiser is done with them anyway.
Example, untested (since you didn't provide an SQLFiddle or sample data) but should work:
BEGIN;
CREATE TEMPORARY TABLE newjoinvalues(
first_id integer,
second_id integer,
primary key(first_id,second_id)
);
-- Now populate `newjoinvalues` with multi-valued inserts or COPY
COPY newjoinvalues(first_id, second_id) FROM stdin;
LOCK TABLE myjoinvalues IN EXCLUSIVE MODE;
INSERT INTO myjoinvalues
SELECT n.first_id, n.second_id
FROM newjoinvalues n
LEFT OUTER JOIN myjoinvalues m ON (n.first_id = m.first_id AND n.second_id = m.second_id)
WHERE m.first_id IS NULL AND m.second_id IS NULL;
COMMIT;
This won't update existing values, but you can do that fairly easily too by using with a second query that does an UPDATE ... FROM while still holding the write table lock.
Note that the lock mode specified above will not block SELECTs, only writes like INSERT, UPDATE and DELETE, so queries can continue to be made to the table while the process is ongoing, you just can't update it.
If you can't accept that an alternative is to run the update in SERIALIZABLE isolation (only works properly for this purpose in Pg 9.1 and above). This will result in the query failing whenever a concurrent write occurs so you have to be prepared to retry it over and over and over again. For that reason it's likely to be better to just live with locking the table for a while.
I am taking my first stumbling steps into DB aware controls (any good tutorials?).
I have a MySql table with 6 columns and have managed to load it into a TDbGrid.
One of the columns however is an index into another table. It is a bar code and, rather than display that, I would like to display the product name associated with it.
How do I do that?
(and can I hide the "gutter" (?) down the left whcih shows the current row?)
Thanks
You should always perform a join from the SQL side, it's much easier then doing it programaticaly
Such as:
SELECT mytable.id, mytable.column1, another_table.barcode
FROM mytable
JOIN another_table ON another_table.id = mytable.barcode_id
To remove gutter you need to uncheck the DBGrid property dgIndicator in Options.
As for "DB-Aware controls" you should try delphi help.
Instead of a table, make use of a query. Then, use a join to select the product name with it, like this:
SELECT
t.*,
p.name
FROM
YourTable t
INNER JOIN Product p on p.barcode = t.barcode
I use t.*, because I don't know the exact columns. In practise, I would not use select *, but specify specific columns instead. If you are going to use * anyway, you can hide speicfic columns by setting the Visible property of the TField object in the dataset/query to False.
I don't know which components you are using to connect to the table, but most of them do have a query-counterpart that allows you to insert SQL instead of a table name.
The gutter can be hidden by going to the property Options in the object inspector, expand it, and set dgIndicator to False.
Just for the record: with ISAM databases like Paradox and DBF typical solution would be so-called master-detail tables relations and it still might work for SQL. Though it would be very inefficient and slow. You'd definitely read som books about SQL.
Use a TQuery component instead of a TTable and set SQL property using the suggested select statements above. If you just add the columns you want to display in your sql statement, you get the result as expected. As for "gutter" you would have to hack the grid in some way at runtime.