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In SQL, we have tables containing individual rows. Tables have strict schemas that every row must obey; there are columns with types and other possible constraints: unique, not null, value checks or references to rows of other tables. Referential integrity lies at the heart of this data approach - guarantee that if row B1 of table B references row A1 of table A, referred row (A1) must exist; orphan rows are not allowed. If we want to delete A1 row, there are two options:
。业内人士推荐体育直播作为进阶阅读
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Troops found the submarine in a mangrove swamp in the Cayapas–Mataje nature reserve.
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