**links** table structure
=========================

The links table holds all the links available in the aequilibrae network model
regardless of the modes allowed on it.

All information on the fields a_node and b_node correspond to a entries in
the node_id field in the nodes table. They are automatically managed with
triggers as the user edits the network, but they are not protected by manual
editing, which would break the network if it were to happen.

The **modes** field is a concatenation of all the ids (mode_id) of the models allowed
on each link, and map directly to the mode_id field in the **Modes** table. A mode
can only be added to a link if it exists in the **Modes** table.

The **link_type** corresponds to the *link_type* field from the *link_types* table.
As it is the case for modes, a link_type can only be assigned to a link if it exists
in the **link_types** table.

The fields **length**, **node_a** and **node_b** are automatically
updated by triggers based in the links' geometries and node positions. Link length
is always measured in **meters**.

The table is indexed on **link_id** (its primary key), **node_a** and **node_b**.

.. csv-table:: 
   :header: "Field", "Type", "NULL allowed", "Default Value"
   :widths:    30,     20,         20,          20

   ogc_fid*,INTEGER,YES,
   link_id,INTEGER,NO,
   a_node,INTEGER,YES,
   b_node,INTEGER,YES,
   direction,INTEGER,NO,0
   distance,NUMERIC,YES,
   modes,TEXT,NO,
   link_type,TEXT,YES,
   line_id,TEXT,YES,
   stop_id,TEXT,YES,
   line_seg_idx,INTEGER,YES,
   trav_time,NUMERIC,NO,
   freq,NUMERIC,NO,
   o_line_id,TEXT,YES,
   d_line_id,TEXT,YES,
   transfer_id,TEXT,YES,
   geometry,LINESTRING,NO,''


(* - Primary key)



The SQL statement for table and index creation is below.


::

   
   
   CREATE TABLE if not exists links (ogc_fid         INTEGER PRIMARY KEY,
                                     link_id         INTEGER NOT NULL UNIQUE,
                                     a_node          INTEGER,
                                     b_node          INTEGER,
                                     direction       INTEGER NOT NULL DEFAULT 0,
                                     distance        NUMERIC,
                                     modes           TEXT    NOT NULL,
                                     link_type       TEXT    REFERENCES link_types(link_type) ON update RESTRICT ON delete RESTRICT,
                                     line_id         TEXT,
                                     stop_id         TEXT    REFERENCES stops(stop) ON update RESTRICT ON delete RESTRICT,
                                     line_seg_idx    INTEGER,
                                     trav_time       NUMERIC  NOT NULL,
                                     freq            NUMERIC  NOT NULL,
                                     o_line_id       TEXT,
                                     d_line_id       TEXT,
                                     transfer_id     TEXT
                                     CHECK(TYPEOF(link_id) == 'integer')
                                     CHECK(TYPEOF(a_node) == 'integer')
                                     CHECK(TYPEOF(b_node) == 'integer')
                                     CHECK(TYPEOF(direction) == 'integer')
                                     CHECK(LENGTH(modes)>0)
                                     CHECK(LENGTH(direction)==1));
   
   select AddGeometryColumn( 'links', 'geometry', 4326, 'LINESTRING', 'XY', 1);
   
   CREATE UNIQUE INDEX idx_link ON links (link_id);
   
   SELECT CreateSpatialIndex( 'links' , 'geometry' );
   
   CREATE INDEX idx_link_anode ON links (a_node);
   
   CREATE INDEX idx_link_bnode ON links (b_node);
   
   CREATE INDEX idx_link_modes ON links (modes);
   
   CREATE INDEX idx_link_link_type ON links (link_type);
   
   CREATE INDEX idx_links_a_node_b_node ON links (a_node, b_node);
   
   INSERT INTO 'attributes_documentation' (name_table, attribute, description) VALUES('links','link_id', 'Unique link ID');
   INSERT INTO 'attributes_documentation' (name_table, attribute, description) VALUES('links','a_node', 'origin node for the link');
   INSERT INTO 'attributes_documentation' (name_table, attribute, description) VALUES('links','b_node', 'destination node for the link');
   INSERT INTO 'attributes_documentation' (name_table, attribute, description) VALUES('links','direction', 'Flow direction allowed on the link');
   INSERT INTO 'attributes_documentation' (name_table, attribute, description) VALUES('links','distance', 'length of the link');
   INSERT INTO 'attributes_documentation' (name_table, attribute, description) VALUES('links','modes', 'modes allowed on the link');
   INSERT INTO 'attributes_documentation' (name_table, attribute, description) VALUES('links','link_type', 'Link type');
   INSERT INTO 'attributes_documentation' (name_table, attribute, description) VALUES('links','line_id', 'ID of the line the link belongs to');
   INSERT INTO 'attributes_documentation' (name_table, attribute, description) VALUES('links','stop_id', 'ID of the stop the link belongss to ');
   INSERT INTO 'attributes_documentation' (name_table, attribute, description) VALUES('links','line_seg_idx', 'Line segment index');
   INSERT INTO 'attributes_documentation' (name_table, attribute, description) VALUES('links','trav_time', 'Travel time');
   INSERT INTO 'attributes_documentation' (name_table, attribute, description) VALUES('links','freq', 'Frequency of link traversal');
   INSERT INTO 'attributes_documentation' (name_table, attribute, description) VALUES('links','*_line_id', 'Origin/Destination line ID for transfer links');
   INSERT INTO 'attributes_documentation' (name_table, attribute, description) VALUES('links','transfer_id', 'Transfer link ID');