Creating and Modifying Data
Last updated on 2023-05-08 | Edit this page
- How can I create, modify, and delete tables and data?
- Write statements that create tables.
- Write statements to insert, modify, and delete records.
So far we have only looked at how to get information out of a database, both because that is more frequent than adding information, and because most other operations only make sense once queries are understood.
Visited tables from the
were used during the earlier episodes. We’re going to build a new
database over the course of the upcoming episodes. Exit the
SQLite interactive session if you’re still in it.
SQLite3 and create a new database, lets call it
newsurvey.db. We use a different name to avoid confusion
with the currently existing
$ sqlite3 newsurvey.db
.mode column and
commands again if you aren’t using the
(Note if you exited and restarted SQLite3 your settings will change back
to the default)
mode column .header on.
If we want to create and modify data, we need to know two other sets of commands.
The first pair are
While they are written as two words, they are actually single commands.
The first one creates a new table; its arguments are the names and types
of the table’s columns. For example, the following statements create the
four tables in our survey database:
CREATE TABLE Person(id text, personal text, family text); CREATE TABLE Site(name text, lat real, long real); CREATE TABLE Visited(id integer, site text, dated text); CREATE TABLE Survey(taken integer, person text, quant text, reading real);
We can get rid of one of our tables using:
DROP TABLE Survey;
Be very careful when doing this: if you drop the wrong table, hope that the person maintaining the database has a backup, but it’s better not to have to rely on it.
Different database systems support different data types for table columns, but most provide the following:
|INTEGER||a signed integer|
|REAL||a floating point number|
|TEXT||a character string|
|BLOB||a “binary large object”, such as an image|
Most databases also support Booleans and date/time values; SQLite uses the integers 0 and 1 for the former, and represents the latter as discussed earlier. An increasing number of databases also support geographic data types, such as latitude and longitude. Keeping track of what particular systems do or do not offer, and what names they give different data types, is an unending portability headache.
When we create a table, we can specify several kinds of constraints
on its columns. For example, a better definition for the
Survey table would be:
CREATE TABLE Survey( integer not null, -- where reading taken taken -- may not know who took it person text, not null, -- the quantity measured quant text real not null, -- the actual reading reading primary key(taken, person, quant), -- key is taken + person + quant foreign key(taken) references Visited(id), foreign key(person) references Person(id) );
Once again, exactly what constraints are available and what they’re called depends on which database manager we are using.
Once tables have been created, we can add, change, and remove records
using our other set of commands,
Here is an example of inserting rows into the
INSERT INTO Site (name, lat, long) VALUES ('DR-1', -49.85, -128.57); INSERT INTO Site (name, lat, long) VALUES ('DR-3', -47.15, -126.72); INSERT INTO Site (name, lat, long) VALUES ('MSK-4', -48.87, -123.40);
We can also insert values into one table directly from another:
CREATE TABLE JustLatLong(lat real, long real); INSERT INTO JustLatLong SELECT lat, long FROM Site;
Modifying existing records is done using the
statement. To do this we tell the database which table we want to
update, what we want to change the values to for any or all of the
fields, and under what conditions we should update the values.
For example, if we made a mistake when entering the lat and long
values of the last
INSERT statement above, we can correct
it with an update:
UPDATE Site SET lat = -47.87, long = -122.40 WHERE name = 'MSK-4';
Be careful to not forget the
WHERE clause or the update
statement will modify all of the records in the
Deleting records can be a bit trickier, because we have to ensure
that the database remains internally consistent. If all we care about is
a single table, we can use the
DELETE command with a
WHERE clause that matches the records we want to discard.
For example, once we realize that Frank Danforth didn’t take any
measurements, we can remove him from the
Person table like
DELETE FROM Person WHERE id = 'danforth';
But what if we removed Anderson Lake instead? Our
table would still contain seven records of measurements he’d taken, but
that’s never supposed to happen:
Survey.person is a foreign
key into the
Person table, and all our queries assume there
will be a row in the latter matching every value in the former.
This problem is called referential integrity:
we need to ensure that all references between tables can always be
resolved correctly. One way to do this is to delete all the records that
'lake' as a foreign key before deleting the record that
uses it as a primary key. If our database manager supports it, we can
automate this using cascading
delete. However, this technique is outside the scope of this
Many applications use a hybrid storage model instead of putting everything into a database: the actual data (such as astronomical images) is stored in files, while the database stores the files’ names, their modification dates, the region of the sky they cover, their spectral characteristics, and so on. This is also how most music player software is built: the database inside the application keeps track of the MP3 files, but the files themselves live on disk.
UPDATE Survey SET person = 'unknown' WHERE person IS NULL;
SQLite has several administrative commands that aren’t part of the
SQL standard. One of them is
.dump, which prints the SQL
commands needed to re-create the database. Another is
.read, which reads a file created by
restores the database. A colleague of yours thinks that storing dump
files (which are text) in version control is a good way to track and
manage changes to the database. What are the pros and cons of this
approach? (Hint: records aren’t stored in any particular order.)
- A version control system will be able to show differences between versions of the dump file; something it can’t do for binary files like databases
- A VCS only saves changes between versions, rather than a complete copy of each version (save disk space)
- The version control log will explain the reason for the changes in each version of the database
- Use CREATE and DROP to create and delete tables.
- Use INSERT to add data.
- Use UPDATE to modify existing data.
- Use DELETE to remove data.
- It is simpler and safer to modify data when every record has a unique primary key.
- Do not create dangling references by deleting records that other records refer to.