Before this, a syntax error would be returned. INTO can also be used.įrom MariaDB 10.4.0, parentheses can be used to specify precedence. Placing the clause after any other SELECT will result in a syntax error. If the last SELECT statement specifies INTO DUMPFILE or INTO OUTFILE, the entire result of the UNION will be written. Individual SELECTs cannot be written INTO DUMPFILE or INTO OUTFILE. Applying to a later SELECT results in a syntax error: ERROR 1234 (42000): Incorrect usage/placement of 'HIGH_PRIORITY' If applied to the first SELECT, it will be ignored. Specifying a query as HIGH_PRIORITY will not work inside a UNION. If the columns retrieved by individual SELECT statements have an alias (AS), the ORDER BY must use that alias, not the real column names. The UNION can have global ORDER BY and LIMIT clauses, which affect the whole resultset. However, this does not affect the order of the UNION, so they only are useful to limit the record read by one SELECT. In this case, the individual queries need to be wrapped between parentheses. Individual SELECTs can contain their own ORDER BY and LIMIT clauses. Since MariaDB 10.1.1, the server can in most cases execute UNION ALL without creating a temporary table, improving performance (see MDEV-334). Until MariaDB 10.1.1, all UNION ALL statements required the server to create a temporary table. In this case, UNION DISTINCT will override any UNION ALLs to its left. UNION ALL and UNION DISTINCT can both be present in a query.
The DISTINCT keyword (the default if the keyword is omitted) causes duplicate rows to be removed by the results. The ALL keyword causes duplicate rows to be preserved.
UNION queries cannot be used with aggregate functions.ĮXCEPT and UNION have the same operation precedence and INTERSECT has a higher precedence, unless running in Oracle mode, in which case all three have the same precedence. See Identifier Qualifiers for syntax details. This permits writing UNIONs which involve multiple databases. Note that currently this is not the case for recursive CTEs - see MDEV-12325. If they don't, the type and length of the columns in the result take into account the values returned by all of the SELECTs, so there is no need for explicit casting. (For example, the first column selected by the first statement should have the same type as the first column selected by the other statements.) Selected columns listed in corresponding positions of each SELECT statement should have the same data type. The column names from the first SELECT statement are used as the column names for the results returned. UNION is used to combine the results from multiple SELECT statements into a single result set.
0 kommentar(er)
