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Recipe #5:
Creating a new table (and related paraphernalia)

2011 January 28

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You are now well conscious that SQL overall performance and efficiency strongly depend on the underlying database layout, i.e. the following design choices are critical:
  • defining tables (and columns) in the most appropriate way.
  • identifying relations connection different tables.
  • supporting often-used relations with an appropriate index.
  • identifying useful constraints, so to preserve data consistency and correctness as much as possible.
It's now time to examine in deeper detail such topics.

Pedantic note: in DBMS/SQL own jargon all this is collectively defined as DDL [Data Definition Language], and is intended as opposed to DML [Data Manipulation Language], i.e. SELECT, INSERT and so on.

CREATE TABLE peoples (
  first_name TEXT,
  last_name TEXT,
  age INTEGER,
  gender TEXT,
  phone TEXT);
This statement will create a very simple table named peoples:

CREATE TABLE peoples2 (
  id INTEGER NOT NULL
    PRIMARY KEY AUTOINCREMENT,
  first_name TEXT NOT NULL,
  last_name TEXT NOT NULL,
  age INTEGER
    CONSTRAINT age_verify
      CHECK (age BETWEEN 18 AND 90),
  gender TEXT
    CONSTRAINT gender_verify
      CHECK (gender IN ('M', 'F')),
  phone TEXT);
This one is more sophisticated version of the same table:

about SQLite data-types


Very shortly said: SQLite hasn't data-types at all ...
You are absolutely free to insert any data-type on every column: the column declared data-type simply have a decorative role, but isn't neither checked not enforced at all.
This one absolutely is not a bug: it's more a peculiar design choice.
Anyway, any other different DBMS applies strong data-type qualification and enforcement, so the SQLite's own behavior may easily look odd and puzzling. Be warned.

Anyway SQLite internally supports the following data-types:
  • NULL: no value at all.
  • INTEGER: actually 64bit integers, so to support really huge values.
  • DOUBLE: floating point, double precision.
  • TEXT: any UTF-8 encoded text string, of unconstrained arbitrary length.
  • BLOB: any generic Binary Long Object, of unconstrained arbitrary length.
Remember: each single cell (row/column intersection) can store any arbitrary data-type.
One unique exception exists: columns declared as INTEGER PRIMARY KEY absolutely require integer values.


ALTER TABLE peoples2
  ADD COLUMN cell_phone TEXT;
You can add any further column even after the initial table creation.

Yet another SQLite's own very peculiar design choice.
  • dropping columns is unsupported.
  • renaming columns is unsupported.
i.e. once you've created a column there is no way at all to change its initial definition.

ALTER TABLE peoples2
  RENAME TO peoples_ok;
Anyway you are absolutely free to change the table name.

DROP TABLE peoples;
And this will completely erase the table (and its whole content) from the DB.

CREATE INDEX idx_peoples_phone
  ON peoples_ok (phone);
This will create an Index.

DROP INDEX idx_peoples_phone;
And this will destroy the same Index.

CREATE UNIQUE INDEX idx_peoples_name
  ON peoples_ok (last_name, first_name);


PRAGMA table_info(peoples_ok);

cid name type notnull dflt_value pk
0 id INTEGER 1 NULL 1
1 first_name TEXT 1 NULL 0
2 last_name TEXT 1 NULL 0
3 age INTEGER 0 NULL 0
4 gender TEXT 0 NULL 0
5 phone TEXT 0 NULL 0
6 cell_phone TEXT 0 NULL 0
You can use PRAGMA table_info(...) in order to query a table layout.

PRAGMA index_list(peoples_ok);

seq name unique
0 idx_peoples_phone 0
1 idx_peoples_name 1

PRAGMA index_info(idx_peoples_name);

seqno cid name
0 2 last_name
1 1 first_name
And using PRAGMA index_list(...) and PRAGMA index_info(...) you can easily query the corresponding Index layout.
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CC-BY-SA logo Author: Alessandro Furieri a.furieri@lqt.it
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