SELECT

Sintaxis

SELECT
    [ALL | DISTINCT | DISTINCTROW]
    [HIGH_PRIORITY]
    [STRAIGHT_JOIN]
    [SQL_SMALL_RESULT] [SQL_BIG_RESULT] [SQL_BUFFER_RESULT]
    [SQL_CACHE | SQL_NO_CACHE] [SQL_CALC_FOUND_ROWS]
    select_expr [, select_expr ...]
    [ FROM table_references
      [WHERE where_condition]
      [GROUP BY {col_name | expr | position} [ASC | DESC], ... [WITH ROLLUP]]
      [HAVING where_condition]
      [ORDER BY {col_name | expr | position} [ASC | DESC], ...]
      [LIMIT {[offset,] row_count | row_count OFFSET offset  [ROWS EXAMINED rows_limit] } |
        [OFFSET start { ROW | ROWS }]
        [FETCH { FIRST | NEXT } [ count ] { ROW | ROWS } { ONLY | WITH TIES }] ]
      procedure|[PROCEDURE procedure_name(argument_list)]
      [INTO OUTFILE 'file_name' [CHARACTER SET charset_name] [export_options] |
        INTO DUMPFILE 'file_name' | INTO var_name [, var_name] ]
      [FOR UPDATE lock_option | LOCK IN SHARE MODE lock_option]
export_options:
    [{FIELDS | COLUMNS}
        [TERMINATED BY 'string']
        [[OPTIONALLY] ENCLOSED BY 'char']
        [ESCAPED BY 'char']
    ]
    [LINES
        [STARTING BY 'string']
        [TERMINATED BY 'string']
    ]
lock_option:
    [WAIT n | NOWAIT | SKIP LOCKED]

Description

SELECT se utiliza para recuperar filas de una o más tablas. Puede incluir sentencias UNION y subqueries.

• Cada select_expr expresión indica la columna o dato que se desea recuperar. Debe indicar al menos una expresión. Para más detalles, consulte Select Expressions más abajo.

• La cláusula FROM indica la/s tabla/s desde las que recuperar filas. Utilice un nombre de tabla o una expresión JOIN para relacionar varias tablas. Verlos detalles de JOIN. Si no se utiliza ninguna tabla, se puede indicar FROM DUAL. Esto no es obligatorio, pero se puede utilizar por compatibilidad.

• Cada tabla también se puede indicar como db_name.tabl_name. Así mismo, cada columna se puede indicar como tbl_name.col_name e incluso como db_name.tbl_name.col_name. Esto permite escribir consultas que involucren vaias bases de datos. Para consultar los detalles de la sintaxis, vea Identifier Qualifiers.

• Si se incluye la cláusula WHERE, esta muestra la condición que deben cumplir las filas para ser seleccionadas. where_condition debe ser cierta (TRUE) para que la fila sea devuelta. Si no se incluye una cláusua WHERE, la consulta devuelve todas las filas.

En la cláusula WHERE puede utilizar cualquier función y operador soportado por MariaDB, excepto en las funciones de agregado. Ver Functions and Operators and Functions and Modifiers for use with GROUP BY (agregado).

• Para ordenar los resultados, utilice la cláusula ORDER BY.

• Para restringir los resultados a un determinado número de filas, utilice la cláusula LIMIT. Si desea empezar en una determinada fila, puede indicar esta como un offset.

• Utilice las cláusulas GROUP BY y HAVING para agrupar las filas cuando tienen campos GROUP BY o valores calculados HAVING en común.

SELECT también se puede utilizar para devolver valores calculados sin necesidad de hacer referencia a una tabla.

Select Expressions

A SELECT statement must contain one or more select expressions, separated by commas. Each select expression can be one of the following:

• The name of a column.
• Any expression using functions and operators.
• * to select all columns from all tables in the FROM clause.
• tbl_name.* to select all columns from just the table tbl_name.

When specifying a column, you can either use just the column name or qualify the column name with the name of the table using tbl_name.col_name. The qualified form is useful if you are joining multiple tables in the FROM clause. If you do not qualify the column names when selecting from multiple tables, MariaDB will try to find the column in each table. It is an error if that column name exists in multiple tables.

You can quote column names using backticks. If you are qualifying column names with table names, quote each part separately as `tbl_name`.`col_name`.

If you use any grouping functions in any of the select expressions, all rows in your results will be implicitly grouped, as if you had used GROUP BY NULL.

DISTINCT

A query may produce some identical rows. By default, all rows are retrieved, even when their values are the same. To explicitly specify that you want to retrieve identical rows, use the ALL option. If you want duplicates to be removed from the resultset, use the DISTINCT option. DISTINCTROW is a synonym for DISTINCT. See also COUNT DISTINCT and SELECT UNIQUE in Oracle mode.

INTO

The INTO clause is used to specify that the query results should be written to a file or variable.

• SELECT INTO OUTFILE - formatting and writing the result to an external file.
• SELECT INTO DUMPFILE - binary-safe writing of the unformatted results to an external file.
• SELECT INTO Variable - selecting and setting variables.

The reverse of SELECT INTO OUTFILE is LOAD DATA.

LIMIT

Restricts the number of returned rows. See LIMIT and LIMIT ROWS EXAMINED for details.

LOCK IN SHARE MODE/FOR UPDATE

See LOCK IN SHARE MODE and FOR UPDATE for details on the respective locking clauses.

OFFSET ... FETCH

ORDER BY

Order a resultset. See ORDER BY for details.

PARTITION

Specifies to the optimizer which partitions are relevant for the query. Other partitions will not be read. See Partition Pruning and Selection for details.

PROCEDURE

Passes the whole result set to a C Procedure. See PROCEDURE and PROCEDURE ANALYSE (the only built-in procedure not requiring the server to be recompiled).

SKIP LOCKED

SQL_CALC_FOUND_ROWS

When SQL_CALC_FOUND_ROWS is used, then MariaDB will calculate how many rows would have been in the result, if there would be no LIMIT clause. The result can be found by calling the function FOUND_ROWS() in your next sql statement.

max_statement_time clause

By using max_statement_time in conjunction with SET STATEMENT, it is possible to limit the execution time of individual queries. For example:

SET STATEMENT max_statement_time=100 FOR 
  SELECT field1 FROM table_name ORDER BY field1;

WAIT/NOWAIT

Set the lock wait timeout. See WAIT and NOWAIT.

Examples

SELECT f1,f2 FROM t1 WHERE (f3<=10) AND (f4='y');

See Getting Data from MariaDB (Beginner tutorial), or the various sub-articles, for more examples.

See Also

• Getting Data from MariaDB (Beginner tutorial)
• Joins and Subqueries
• LIMIT
• ORDER BY
• GROUP BY
• Common Table Expressions
• SELECT WITH ROLLUP
• SELECT INTO OUTFILE
• SELECT INTO DUMPFILE
• FOR UPDATE
• LOCK IN SHARE MODE
• Optimizer Hints
• Oracle mode from MariaDB 10.3

13/12/2022 - Traducido por Carlos Ponce de León