三,DISTINCT Select distinct rows using DISTINCT operator. The DISTINCT clause keeps one row for each group of duplicates. If you specify multiple columns, the DISTINCT clause will evaluate the duplicate based on
postgresql 查找PL/PGSQL函数以返回N列如果不在某个地方对列列表进行硬编码（无论是在调用函数时还是在...
* < 0 negative of multiplier for number of rows * The special negative case allows us to cope with columns that are * unique (stadistinct = -1) or nearly so (for example, a column in which * non-null values appear about twice on the average could be represented * by stadistinct = ...
2.3 外键约束 SELECT DISTINCT CONCAT('ALTER TABLE ', kcu.table_name, ' ADD CONSTRAINT ', tc.constraint_name,'_', kcu.column_name, ' FOREIGN KEY (', kcu.column_name, ') REFERENCES ', kcu.referenced_table_name, '(', kcu.referenced_column_name, ');') AS create_statement FROM inf...
must be distinct. An error occurs if you try to add a new row with a key value that matches an existing row. This constraint does not apply to NULL values except for the BDB storage engine. For other engines, a UNIQUE index allows multiple NULL values for columns that can contain NULL...
Combining Multiple Indexes Asingleindex scan can only use query clauses that use the index's columns with operators of its operator class and are joined with AND.Forexample, given an indexon(a, b) a query conditionlikeWHEREa =5ANDb =6could use the index, but a querylikeWHEREa =5ORb ...
Combining Multiple Indexes A single index scan canonlyuse query clauses that use the index's columns with operators of its operator class and are joined with AND. For example, given an index on (a, b) a query condition like WHERE a = 5 AND b = 6 could use the index, but a query...