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Information Schema COLUMNS Table

The Information Schema COLUMNS table provides information about columns in each table on the server.

It contains the following columns:

Column Description Introduced
TABLE_CATALOG Always contains the string 'def'.
TABLE_SCHEMA Database name.
TABLE_NAME Table name.
COLUMN_NAME Column name.
ORDINAL_POSITION Column position in the table. Can be used for ordering.
COLUMN_DEFAULT Default value for the column. From MariaDB 10.2.7, literals are quoted to distinguish them from expressions. NULL means that the column has no default. In MariaDB 10.2.6 and earlier, no quotes were used for any type of default and NULL can either mean that there is no default, or that the default column value is NULL.
IS_NULLABLE Whether the column can contain NULLs.
DATA_TYPE The column's data type.
CHARACTER_MAXIMUM_LENGTH Maximum length.
CHARACTER_OCTET_LENGTH Same as the CHARACTER_MAXIMUM_LENGTH except for multi-byte character sets.
NUMERIC_PRECISION For numeric types, the precision (number of significant digits) for the column. NULL if not a numeric field.
NUMERIC_SCALE For numeric types, the scale (significant digits to the right of the decimal point). NULL if not a numeric field.
DATETIME_PRECISION Fractional-seconds precision, or NULL if not a time data type.
CHARACTER_SET_NAME Character set if a non-binary string data type, otherwise NULL.
COLLATION_NAME Collation if a non-binary string data type, otherwise NULL.
COLUMN_TYPE Column definition, a MySQL and MariaDB extension.
COLUMN_KEY Index type. PRI for primary key, UNI for unique index, MUL for multiple index. A MySQL and MariaDB extension.
EXTRA Additional information about a column, for example whether the column is an invisible column, or, from MariaDB 10.3.6, WITHOUT SYSTEM VERSIONING if the table is not a system-versioned table. A MySQL and MariaDB extension.
PRIVILEGES Which privileges you have for the column. A MySQL and MariaDB extension.
COLUMN_COMMENT Column comments.
IS_GENERATED Indicates whether the column value is generated (virtual, or computed). Can be ALWAYS or NEVER. MariaDB 10.2.5
GENERATION_EXPRESSION The expression used for computing the column value in a generated (virtual, or computed) column. MariaDB 10.2.5

It provides information similar to, but more complete, than SHOW COLUMNS and mysqlshow.

Examples

SELECT * FROM information_schema.COLUMNS\G
...
*************************** 9. row ***************************
           TABLE_CATALOG: def
            TABLE_SCHEMA: test
              TABLE_NAME: t2
             COLUMN_NAME: j
        ORDINAL_POSITION: 1
          COLUMN_DEFAULT: NULL
             IS_NULLABLE: YES
               DATA_TYPE: longtext
CHARACTER_MAXIMUM_LENGTH: 4294967295
  CHARACTER_OCTET_LENGTH: 4294967295
       NUMERIC_PRECISION: NULL
           NUMERIC_SCALE: NULL
      DATETIME_PRECISION: NULL
      CHARACTER_SET_NAME: utf8mb4
          COLLATION_NAME: utf8mb4_bin
             COLUMN_TYPE: longtext
              COLUMN_KEY: 
                   EXTRA: 
              PRIVILEGES: select,insert,update,references
          COLUMN_COMMENT: 
            IS_GENERATED: NEVER
   GENERATION_EXPRESSION: NULL
...

CREATE TABLE t (
  s1 VARCHAR(20) DEFAULT 'ABC',
  s2 VARCHAR(20) DEFAULT (concat('A','B')),
  s3 VARCHAR(20) DEFAULT ("concat('A','B')"),
  s4 VARCHAR(20),
  s5 VARCHAR(20) DEFAULT NULL,
  s6 VARCHAR(20) NOT NULL,
  s7 VARCHAR(20) DEFAULT 'NULL' NULL,
  s8 VARCHAR(20) DEFAULT 'NULL' NOT NULL
);

SELECT 
  table_name, 
  column_name, 
  ordinal_position, 
  column_default,
  column_default IS NULL
FROM information_schema.COLUMNS
WHERE table_schema=DATABASE()
AND TABLE_NAME='t';

From MariaDB 10.2.7:

+------------+-------------+------------------+-----------------------+------------------------+
| table_name | column_name | ordinal_position | column_default        | column_default IS NULL |
+------------+-------------+------------------+-----------------------+------------------------+
| t          | s1          |                1 | 'ABC'                 |                      0 |
| t          | s2          |                2 | concat('A','B')       |                      0 |
| t          | s3          |                3 | 'concat(''A'',''B'')' |                      0 |
| t          | s4          |                4 | NULL                  |                      0 |
| t          | s5          |                5 | NULL                  |                      0 |
| t          | s6          |                6 | NULL                  |                      1 |
| t          | s7          |                7 | 'NULL'                |                      0 |
| t          | s8          |                8 | 'NULL'                |                      0 |
+------------+-------------+------------------+-----------------------+------------------------+

In the results above, the two single quotes in concat(''A'',''B'') indicate an escaped single quote - see string-literals. Note that while mysql-command-line-client appears to show the same default value for columns s5 and s6, the first is a 4-character string "NULL", while the second is the SQL NULL value.

MariaDB 10.2.6 and before:

+------------+-------------+------------------+-----------------+------------------------+
| table_name | column_name | ordinal_position | column_default  | column_default IS NULL |
+------------+-------------+------------------+-----------------+------------------------+
| t          | s1          |                1 | ABC             |                      0 |
| t          | s2          |                2 | concat('A','B') |                      0 |
| t          | s3          |                3 | concat('A','B') |                      0 |
| t          | s4          |                4 | NULL            |                      1 |
| t          | s5          |                5 | NULL            |                      1 |
| t          | s6          |                6 | NULL            |                      1 |
| t          | s7          |                7 | NULL            |                      0 |
| t          | s8          |                8 | NULL            |                      0 |
+------------+-------------+------------------+-----------------+------------------------+
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https://mariadb.com/kb/en/information-schema-columns-table/