The second major function performed by a WHERE clause is to join multiple tables together into a single table that can be queried more easily.
WHERE 子句执行的第二个主要功能是将多个表连接到单个表中,从而更易于查询。
If no WHERE clause is included, all rows in the table are deleted.
如果没有包括任何where子句,则删除表中的所有行。
A more efficient technique is to filter database rows by placing conditions in the WHERE clause, which is evaluated immediately after the tables are specified within the FROM clause.
更有效的方法是通过在where子句中放置条件来过滤数据库行,在from子句中指定表之后会立即对where子句求值。
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