PL/SQL Challenge 每日一题：2016-12-24 非空约束

A kingfisher perches on a lotus seedpod in a lotus pond at Jin'an Lake Park in Jin'an District, Fuzhou, southeast China's Fujian Province, June 21, 2025. Rich in wetland resources, Fuzhou has a wetland area of 194,200 hectares and a wetland rate of 15.14 percent, accounting for about one-third of the wetland area in Fujian Province.

In the practice of wetland protection, the city has invested 30 million yuan (about 4.16 million U.S. dollars) annually returning farmlands to wetlands, and providing employment opportunities to farmers.

Fuzhou was accredited as an international wetland city during the opening of the 15th Meeting of the Conference of the Contracting Parties to the Ramsar Convention on Wetlands (COP15) held in Zimbabwe's resort city of Victoria Falls. (Xinhua/Wei Peiquan)

An aerial drone photo taken on Aug. 1, 2025 shows the landscape around a wetland park in Fuzhou, southeast China's Fujian Province. Rich in wetland resources, Fuzhou has a wetland area of 194,200 hectares and a wetland rate of 15.14 percent, accounting for about one-third of the wetland area in Fujian Province.

In the practice of wetland protection, the city has invested 30 million yuan (about 4.16 million U.S. dollars) annually returning farmlands to wetlands, and providing employment opportunities to farmers.

Fuzhou was accredited as an international wetland city during the opening of the 15th Meeting of the Conference of the Contracting Parties to the Ramsar Convention on Wetlands (COP15) held in Zimbabwe's resort city of Victoria Falls. (Xinhua/Wei Peiquan)

An aerial drone photo taken on July 27, 2025 shows the wetland scenery at the Minjiang River estuary, in southeast China's Fujian Province. Rich in wetland resources, Fuzhou has a wetland area of 194,200 hectares and a wetland rate of 15.14 percent, accounting for about one-third of the wetland area in Fujian Province.

In the practice of wetland protection, the city has invested 30 million yuan (about 4.16 million U.S. dollars) annually returning farmlands to wetlands, and providing employment opportunities to farmers.

Fuzhou was accredited as an international wetland city during the opening of the 15th Meeting of the Conference of the Contracting Parties to the Ramsar Convention on Wetlands (COP15) held in Zimbabwe's resort city of Victoria Falls. (Xinhua/Wei Peiquan)

Colored sandpipers forage for food in Shantang Village of Lianjiang County, Fuzhou, southeast China's Fujian Province, July 30, 2025. Rich in wetland resources, Fuzhou has a wetland area of 194,200 hectares and a wetland rate of 15.14 percent, accounting for about one-third of the wetland area in Fujian Province.

In the practice of wetland protection, the city has invested 30 million yuan (about 4.16 million U.S. dollars) annually returning farmlands to wetlands, and providing employment opportunities to farmers.

Fuzhou was accredited as an international wetland city during the opening of the 15th Meeting of the Conference of the Contracting Parties to the Ramsar Convention on Wetlands (COP15) held in Zimbabwe's resort city of Victoria Falls. (Xinhua/Wei Peiquan)

A gallicrex cinerea flies over a pond at Shantang Village of Lianjiang County, in Fuzhou, southeast China's Fujian Province, July 30, 2025. Rich in wetland resources, Fuzhou has a wetland area of 194,200 hectares and a wetland rate of 15.14 percent, accounting for about one-third of the wetland area in Fujian Province.

In the practice of wetland protection, the city has invested 30 million yuan (about 4.16 million U.S. dollars) annually returning farmlands to wetlands, and providing employment opportunities to farmers.

Fuzhou was accredited as an international wetland city during the opening of the 15th Meeting of the Conference of the Contracting Parties to the Ramsar Convention on Wetlands (COP15) held in Zimbabwe's resort city of Victoria Falls. (Xinhua/Wei Peiquan)

An aerial drone photo taken on July 27, 2025 shows the landscape around the Jin'an Lake Park in Jin'an District, Fuzhou, southeast China's Fujian Province. Rich in wetland resources, Fuzhou has a wetland area of 194,200 hectares and a wetland rate of 15.14 percent, accounting for about one-third of the wetland area in Fujian Province.

In the practice of wetland protection, the city has invested 30 million yuan (about 4.16 million U.S. dollars) annually returning farmlands to wetlands, and providing employment opportunities to farmers.

Fuzhou was accredited as an international wetland city during the opening of the 15th Meeting of the Conference of the Contracting Parties to the Ramsar Convention on Wetlands (COP15) held in Zimbabwe's resort city of Victoria Falls. (Xinhua/Wei Peiquan)

An aerial drone photo taken on July 27, 2025 shows the wetland scenery at the Minjiang River estuary, in southeast China's Fujian Province. Rich in wetland resources, Fuzhou has a wetland area of 194,200 hectares and a wetland rate of 15.14 percent, accounting for about one-third of the wetland area in Fujian Province.

In the practice of wetland protection, the city has invested 30 million yuan (about 4.16 million U.S. dollars) annually returning farmlands to wetlands, and providing employment opportunities to farmers.

Fuzhou was accredited as an international wetland city during the opening of the 15th Meeting of the Conference of the Contracting Parties to the Ramsar Convention on Wetlands (COP15) held in Zimbabwe's resort city of Victoria Falls. (Xinhua/Wei Peiquan)