The problem is how to exclude a certain column from the result of all column selection (SELECT *)? Suppose, we select all columns from the Laptop table:
SELECT * FROM Laptop;
To exclude from the result one column, for example, the code column, one has to specify all other columns of the Laptop table:
SELECT model, speed, ram, hd, price, screen FROM Laptop;
(Usually it is also necessary to enumerate columns in the INSERT statement, omitting autoincremental ones and columns with default values).
It would be quite good if one could write something like
SELECT *[^code] FROM Laptop;
It would let us avoid a routine error-prone work of rewriting column names and would help us to form dynamic queries when a table name neither a column count is known beforehand. Unfortunately, SQL does not provide such means. A script that produces the required list of columns can be written instead. The script would be used in queries then.
It can be done with the help of a special view INFORMATION_SCHEMA.COLUMNS of the system catalog. The view contains all column names for each database table:
SELECT COLUMN_NAME FROM INFORMATION_SCHEMA.COLUMNS WHERE TABLE_NAME='Laptop' AND COLUMN_NAME NOT IN ('code');
The NOT IN predicate is used for the further purpose of excluding a set of columns from the result of queries.
Now a string containing a list of column names separated by commas should be generated. Non-standard means should be used for that purpose.
The task can be fulfilled with the FOR XML PATH clause:
SELECT REPLACE( (SELECT COLUMN_NAME AS 'data()' FROM INFORMATION_SCHEMA.COLUMNS WHERE TABLE_NAME='Laptop'AND COLUMN_NAME NOT IN ('code') ORDER BY ORDINAL_POSITION FOR XML PATH('')) ,' ',', ');
Here the REPLACE function replaces space characters between column names with commas.
In principle, the required SELECT statement can be generated entirely and can be used dynamically in an application code:
SELECT 'SELECT ' +REPLACE( (SELECT COLUMN_NAME AS 'data()' FROM INFORMATION_SCHEMA.COLUMNS WHERE TABLE_NAME='Laptop' AND COLUMN_NAME NOT IN ('code') ORDER BY ORDINAL_POSITION FOR XML PATH('')) ,' ',', ') + ' FROM Laptop';
In MySQL a special column function GROUP_CONCAT can be used:
SELECT GROUP_CONCAT(COLUMN_NAME) FROM INFORMATION_SCHEMA.COLUMNS WHERE TABLE_SCHEMA = 'computers' AND TABLE_NAME='Laptop' AND COLUMN_NAME NOT IN ('code') ORDER BY ORDINAL_POSITION;
It should be mentioned that the information schema in MySQL covers all database server, not certain databases. That is why if different databases contains tables with identical names, search condition of the WHERE clause should specify the schema name: TABLE_SCHEMA='computers'.
Strings are concatenated with the CONCAT function in MySQL. The final solution of our problem can be expressed in MySQL as
SELECT CONCAT('SELECT ', (SELECT GROUP_CONCAT(COLUMN_NAME) FROM INFORMATION_SCHEMA.COLUMNS WHERE TABLE_SCHEMA='computers' AND TABLE_NAME='Laptop' AND COLUMN_NAME NOT IN ('code') ORDER BY ORDINAL_POSITION ), ' FROM Laptop');
Column values can be pivoted into a text list with the two PostgreSQL built-in functions: ARRAY and ARRAY_TO_STRING. The first one transforms a query result into an array, and the second one concatenates array components into a string. List components separator can be specified with the second parameter of the ARRAY_TO_STRING function. The solution can be the following one:
SELECT 'SELECT ' || ARRAY_TO_STRING(ARRAY(SELECT COLUMN_NAME::VARCHAR(50) FROM INFORMATION_SCHEMA.COLUMNS WHERE TABLE_NAME='laptop' AND COLUMN_NAME NOT IN ('code') ORDER BY ORDINAL_POSITION ), ', ') || ' FROM Laptop';
Here strings are concatenated with the standard operator "||". The COLUMN_NAME data type is information_schema.sql_identifier. This data type requires explicit conversion to CHAR/VARCHAR data type.
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