{"id":166,"date":"2017-11-26T11:04:53","date_gmt":"2017-11-26T11:04:53","guid":{"rendered":"https:\/\/blinksql.com\/?p=166"},"modified":"2017-11-26T11:04:53","modified_gmt":"2017-11-26T11:04:53","slug":"constraints-in-postgresql","status":"publish","type":"post","link":"https:\/\/blinksql.com\/?p=166","title":{"rendered":"Constraints in Postgresql"},"content":{"rendered":"

Constraints:<\/strong>
\n->Constraints are the rules enforced on data columns on table.
\n-> These are used to prevent invalid data from being entered into the database. This ensures the accuracy and reliability of the data in the database.
\n->Constraints could be column level or table level.
\n-> Column level constraints are applied only to one column whereas table level constraints are applied to the whole table.<\/p>\n

Primary Key:
\n<\/strong>
\nA primary key is a column or a group of columns that is used to identify a row uniquely in a table.
\nSyntax:<\/strong>
\nALTER TABLE TABLE_NAME ADD PRIMARY KEY (column_1, column_2);
\nEg:<\/strong>
\nAlter table Employee add primary key(emp_id);<\/p>\n

-> If we want a column in a table as primary key and we want the field is auto-incremented by 1 then
\nSyntax:
\n<\/strong>
\nALTER TABLE vendors ADD COLUMN ID SERIAL PRIMARY KEY;<\/p>\n

Drop Primary Key:
\n<\/strong>
\nALTER TABLE TABLE_NAME DROP CONSTRAINT primary_key_constraint_name;<\/p>\n

-> To get the name of the primary key available in a table
\nselect constraint_name from information_schema.table_constraints where table_name = ‘attribute’ and constraint_type = ‘PRIMARY KEY’;
\n-> If we do not specify any name to the contraint by default the name will be tanle_name_pkey<\/p>\n

Foreign Key:
\n<\/strong>
\nA foreign key constraint specifies that the values in a column (or a group of columns) must match the values appearing in some row of another table.
\nSyntax:<\/strong>
\nALTER TABLE manager ADD CONSTRAINT fkey_constraint FOREIGN KEY (user_id) REFERENCES users (uid) ON DELETE CASCADE;
\n-> Here the user_id column of of manager table is referencing the uid column of users table.
\n-> Before any modification in the uid column of the user table we must modify the user_id column of the manager table.<\/p>\n


\nNOT NULL:<\/strong>
\nSyntax:<\/strong>
\nALTER TABLE table_name ALTER COLUMN column_name SET NOT NULL;<\/p>\n

Note: Before adding not null constraint to the alredy existing table make sure to update the existing data in the column to be not null.
\n
\nDefault:<\/strong>
\n-> To set a value by default this constraint should be used.
\nSyntax:
\n<\/strong>
\nAlter Table Table_name alter column_name set DEFAULT ‘TC’::character varying;
\nEg:
\nALTER TABLE ONLY service ALTER COLUMN service_type SET DEFAULT ‘DATA’;<\/p>\n

Unique:<\/strong>
\n-> It is used to make sure that every row in the column must have an unique value.
\nSyntax:
\nALTER TABLE table_name
\nADD CONSTRAINT constraint_name UNIQUE (column1, column2, … column_n);
\nEg:
\nADD CONSTRAINT unique_key UNIQUE (name,type,description);<\/p>\n

Drop Unique :<\/strong>
\nALTER TABLE attribute drop CONSTRAINT unique_key;<\/p>\n","protected":false},"excerpt":{"rendered":"

Constraints: ->Constraints are the rules enforced on data columns on table. -> These are used to prevent invalid data from being entered into the database. This ensures the accuracy and…<\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_eb_attr":""},"categories":[1],"tags":[],"yoast_head":"\nConstraints in Postgresql - Blink SQL<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/blinksql.com\/?p=166\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Constraints in Postgresql - Blink SQL\" \/>\n<meta property=\"og:description\" content=\"Constraints: ->Constraints are the rules enforced on data columns on table. -> These are used to prevent invalid data from being entered into the database. 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